Billionnairedom, or at least millionnairedom, awaits the man or woman who unravels this dilemma:

Cars are getting smaller each year, but men are not growing shorter. Specialty stores fill the clothing needs of big and tall men. Why not have a specialty automobile dealership to put the big man--without bumping his head--behind the wheel?

My obsession with car dimensions began three months ago when I started looking for a new car. I bought my present car new in 1973. I had a hard time then finding a car I could fit into, but I had some choice. I wound up with a car that put four inches between the top of my head and the roof.

I was envious of older men who told me that in earlier decades they had been able to wear a hat and still have six or eight inches of glorious space before reaching the roof. This year, after looking at almost every car made, I was thrilled when I found one in which I could wedge an index finger between my head and the roof.

I am not a giraffe or a professional basketball player. My height is 6 feet, 4 inches. When I look across the church when we are all standing, I notice a lot of men (and a few women) bigger than I am.

About 15 car salesmen told me about a television commercial --speaking of basketball players --that showed 7-foot-4-inch hoop stars fitting comfortably into cars whose main claim to fame is their smallness. How do they do it? One theory is that they have one or two sub-knees, so they can fold up their legs accordion-style.

The explanation I now accept (more or less) for my inability to fit into cars that are no problem for basketball players: I have a disproportionately long torso and thus disproportionately short legs. That is, the distance from my--how shall I put it--bottom (not my feet) and the top of my head is greater than that for TV car-commercial types.

Actually, I was not aware that I had such unusual proportions before I tried to find a new car. Opportunities for heightened self-awareness are everywhere.

Car salesmen were not without suggestions on how I might overcome my difficulty. "Slouch a little more, sir," was a piece of advice I received several times. Often salesmen rolled the seat back further and further in an effort to fit my head under the roof of the car. Sometimes I was so far back that I could have put my head on the shoulder of the person in the back seat, had there been a person in the back seat. This might be pleasant some of the time, but hardly the basis for buying a car.

When car salesmen saw they could not fit me into their cars comfortably, they told me to l) buy a car with a sun roof (some call them moon roofs) and stick my head out the top. 2) think the three "T's": tank, truck, taxi.

The April 1981 Consumer Reports' analysis of most cars, and their chart on interior dimensions, provides no comfort to squashed and bent prospective car buyers.

The greatest number of inches their 5-foot-9 investigator found above his head was five. This means there is no car made in which a man 6-feet-2 or greater can hold up his head?

Regulations designed to increase gas mileage have produced a new class of the underprivileged: the oversized.

Some suggestions:

1. Regulations should be modified to allow for one "big person's car." This design could actually save gas, since men who cannot find cars that fit are apt to buy trucks or vans.

2. Auto makers in Detroit, Tokyo, and elsewhere might consider redesigning--or reverting to original designs--to accommodate big people. The old Volkswagon Bug, for example, is fairly comfortable.

3. Automobile companies using tall men in advertisements should be required to state their models' torso/leg proportions.

4. A system for labeling car sizes-- similar to suit sizes--should be devised. Think how long it would take to buy a suit if the 38 Regulars were mixed up with the 42 Extra Longs.

Practical solutions:

Try a Chrysler Le Baron or a Dodge Diplomat (particularly station wagons), which have a bit more room than Consumer Reports gives them credit for; a Checker Marathon; a special lower seat, which some dealers will install. Or try good older cars.

And me? I put out the money necessary to fix up my old car. Bumps in the road will not be reflected with bumps on my head.