Selecting the top 100 businesses in Washington ought to be a snap.

You line up the local companies with the big ones at one end and the little ones at the other and start counting from the top down: Martin Marietta, C&P Telephone, Mars Inc., Marriott, Southern Railway, and so on. When you get to a hundred, that's it.

Ah, if it were that simple.

Instead the assignment turns out to be as tricky as judging a kid's pet show. How do you compare a pedigreed pooch with the only frog on the block? What do you mean by big? Biggest in sales? Biggest in assets? Biggest in profits? Biggest in employes?

Take the three Washington firms that everybody agrees are the biggest locally based stock brokers--Johnston Lemon & Co., Ferris & Co. and Folger Nolan Fleming Douglas Inc. Folger Nolan has been around longer and has more assets. Johnston Lemon leads in underwritings. Ferris has more brokers.

Commission income, retained earnings, assets under management--several criteria can be used and any of them can be challenged, one broker acknowledged. And any ranking of Washington-based brokers leaves out the vast volume of stock and commodity trading handled by the local offices of Merrill Lynch, Dean Witter and other Wall Street firms.

Then there's the problem of comparing a broker to a high tech research outfit or a real estate firm.

No one ranks brokers on the basis of the total value of the securities handled. If that were done, the commodity futures brokers would win hands down. Treasury bill futures contracts are worth $1 million apiece and plenty of local speculators trade them by the handful. But only a fraction of that contract value is put up in cash and only a fraction of that stays in the broker's hands as commission. No one seriously argues that a broker who handles better than $1 billion worth of T-bill futures is as big an outfit as Best Products, whose showroom sales topped $1 billion last year.

And what about real estate brokers? Shannon & Luchs is clearly number one among greater Washington realtors, handling sales of property worth perhaps three quarters of a billion dollars even in a bad year like this one.

Now, $750 million in real estate sales is a "more real" number than $750 million worth of T-bill futures, but it's not the same as People's Drug Stores, Geico Corp. or The Washington Post Co. reporting revenues in the $750 million range.

The revenues of stock and real estate brokers are more like those of advertising agencies. Some ad agencies like to measure themselves by the size of their billings, which reflect the total value of all the ads they place in a year.

Advertising Age, however, ranks agencies primarily on the basis of their gross income--what they get to keep. While billings of U.S. agencies were $35 billion last year, gross income was more like $5 billion.

On a gross income basis, Ad Age says the biggest local agency was Ehrlich Manes & Associates, at $4 million last year, while Earle Palmer Brown & Associates took in $3,675,000.

But total billings of those agencies are estimated to be in the same range as the $30 million annual revenues of the smallest companies in this year' Top 100--outfits like STSC Inc., Advanced Technology Inc. and C-3 Inc.

One of the ways we've handled the apples and oranges problem of comparing different kinds of companies is to rank firms by industry, so judgments are made between similar businesses on the basis of revenues or assets.

Even that is tricky. Compared to other local financial institutions, International Bank looks tiny. It's revenues of only $20.4 million are not only last in the finance catergory, they are the least in the Top 100.

But IB has assets of $235 million--more than any local research, service or consulting firm--and 5,000 employes scattered all over the world. As a unique species of investment company, International Bank counts its revenues differently than anyone else in Washington business. What IB reports as revenues are its share of the profits of the more than dozen major companies in which it has invested.

Translating the financial reporting quirks of various kinds of companies is a minor problem compared to getting a fix on the privately-owned companies that do not make any financial data public.

This year we've grouped the private firms together and given estimates of sales, assets and other financial details only where data of established reliability is available.

Trade publications sometimes have pretty good statistics, as in the case of Advertising Age. McGraw Hill's Engineering News Record puts out data on construction companies, but the 1981 figures are not available yet and revenues in that business can vary widely from year to year.

Competitors, industry sources and back-channel connections to the companies themselves provided some of the estimates. These sources can produce very good estimates even for compulsively private firms like Mars Inc. Industry estimates that Mars is a $3 billion-a-year operation are solid enough that there's no doubt the company ranks in the top three or four Washington corporations. But how much construction work did the Howard P. Foley Co. do last year? Or Blake Construction, the Oliver T. Carr Co., or Weaver Brothers?

Finally there are the companies that have a major impact on the economy of the Washington area but are not based here. Mobil Oil has multitudes on the payroll at Reston, but since its home office is in New York, Mobil isn't counted. Nor is IBM, which has some 10,000 workers in the Washington area.

Factor those companies into the equation and its obvious that even a listing of the Top 100 businesses in Washington gives only a fragmentary picture of the area's diverse economy.