For 1989, the Cadillac DeVille takes a step backwards, but it is a step backwards that will no doubt please traditional Cadillac buyers.

In reversing a trend of downsizing, the new DeVille is larger than the previous model. It has a longer wheelbase, longer length, roomier interior and larger trunk - all on a classy chassis.

The DeVille's body has also been redesigned, but it is really not that apparent since it is doubtful if it could be mistaken for anything but a Cadillac. The ''new'' look is somewhat aerodynamic with restyled fenders and quarter panels, hood and trunk, grille, bumpers and front and rear fascia. Again, because it is so Cadillac-looking, you may have to look it over to see the changes.

As a side note, it might be interesting to learn that the DeVille is now 40 years old. Introduced in 1949 as a hardtop coupe, more than 4.6 million DeVilles have been sold, making it the best selling model in Cadillac's 87- year old history.

The Sedan DeVille test car (supplied by Daniels Cadillac-BMW, 1327 Tilghman St., Allentown) had all those attributes Cadillac is known for. It was comfortable, quiet, smooth and loaded with luxury. But since this is what you are paying for, this is what you should expect.

And speaking of price, there once was a time when Cadillac was the most expensive car in this country. This, of course, is no longer true. It didn't become any cheaper but a good number of imports became that much more expensive. And not just the big luxury models and sports cars, but many of the upscale small cars.

Anyway, new dimensions for the Sedan DeVille include a wheelbase of 113.8 inches, length of 205.6 inches, width of 72.5 inches, height of 55 inches and curb weight of 3,470 pounds. Compared to last year's model, this translates into a three-inch increase in wheelbase and nine-inch increase in length, with only a negligible increase in weight.

The new Sedan DeVille easily qualifies as a large car (an interior index of 120 cubic feet or more) with its impressive 136 cubic feet EPA index, a 114 cubic feet passenger compartment and a cargo area of 18 cubic feet. This breaks down to an increase of four cubic feet in the passenger compartment and two in the trunk.

It is a six-passenger car with plenty of room for everybody and room for their luggage. And passengers, and even their luggage, will be surrounded by luxury. Cadillac certainly doesn't skimp on anything.

With all of its power equipment and four-speed automatic transmission, the Sedan DeVille is an easy car to drive. Sure its a little long and parking and weaving in and out of traffic takes a little more judgment. But not all that much. Instruments and controls are nicely laid out and located where you would expect to find them. New features include an oil life indicator that reminds owners to change oil and an express-down driver's window.

A very convenient fea ture is ''retained accessory power,'' which provides electrical power from the time the key is turned off until the door is opened. Remember turning the key off, putting it in your pocket and then discovering a power window or sunroof is open. Well, with this system, you have a second chance.

Handling is predictable and probably better than most people would think. This front-wheel-drive car features four-wheel independent suspension, electronic level control, superlift air adjustable rear struts and front and rear stabilizer bars. Everything is also helped along by new deflective disk shock absorbers, which, according to Cadillac, provides a soft, comfortable ride while maintaining a highly-damped road feel and the increased ability to absorb and dissipate sudden road shocks.

In other words, here's a car that really has the ability to run the roads, though it is doubtful if it is going to be put to the test many times. But it should be comforting to kno there is more than enough suspension to handle the job. (Although the test car was not so equipped, the DeVille is available with ABS - anti-lock braking system - , certainly an option well worth thinking about. Another safety option is a driver's supplemental inflatable restraint system located in the steering wheel.)

Powering the DeVille is Cadillac's 4.5-liter/273-cubic-inch V-8, which, for what it is worth, makes Cadillac the only auto manufacturer in the world producing front-wheel drive V-8 powered automobiles. Cadillac and V-8s do go back a long way. In fact back to 1915 when the company offered the first production car with a V-type water-cooled eight. Ever since there has always been a V-8 in a Cadillac (there were also V-6s, V-12s and V-16s).

There is really nothing new about the 4.5 V-8 this year, which is somewhat understandable since the engine (originally a 4.1-liter) was extensively reworked for the 1988 model year. In addition to an increase in displacement, it produced more horsepower and torque through all driving ranges.

This digital fuel-injected, overhead valve engine is somewhat unusual in that it has an aluminum block with cast-iron cylinder liners. But it certainly does run. Rated at 155 horsepower at 4,000 rpm and 240 foot pounds torque at 2,800 rpm, the 4.5 V-8 moved the test car from 0-60 mph in a shade over 10 seconds, really quick for a luxury car.

The four-speed automatic transmission, featuring a viscous converter clutch, has been re-tuned this year for smoother operation with less hunting and busyness. Another interesting feature of the unit is an electronic torque management system that limits torque, particularly during hard accelerations, to reduce the possibility of overstressing the transmission.

Fuel mileage was quite decent for a large car. The test car averaged 15 miles per gallon for city driving and 21 mpg over the highway.

Base price for the Sedan DeVille is $25,435. Standard equipment includes climate control, power windows and door locks, computer diagnostics, tilt and telescopic steering wheel, AM/FM stereo with cassette and clock, 6-way power driver's seat, power trunk deck release, outside temperature display, power outside mirrors and soft ray glass.

With options totaling $1,445 and a destination charge of $550, total price on the test car came to $27,430. Options included Option Package D (door edge guards, floor mats, remote release fuel filler door, illuminated entry system, illuminated vanity mirrors, 6-way power passenger seat, trumpet horn, trunk lid pulldown and twilight sentinel) $894; rear window defogger, $170, and locking wire wheel disks, $320.

The DeVille is protected by Cadillac's very strong ''Bumper to Bumper Plus'' warranty that provides four-year/50,000-mile coverage on all major systems including the powertrain, air conditioning, steering and suspension, electronics and power o ptions. Corrosion protection is six years/100,000 miles, and emission system components are covered for 50,000 miles. In addition, the DeVille, as well as any other Cadillac - new or old - qualifies for Cadillac Roadside Service.