The vast majority of top firms tout their global capabilities. But what that actually means is not very clear. It doesn't mean they are in every country, and it certainly doesn't mean they are evenly spread across the globe.

So how large should a truly global law firm be in each of the world's regions? Should it have more lawyers based in the U.S. legal market than in Asia or Europe?

These are difficult questions. Simply looking at the size of each economy is not particularly helpful. The Asia-Pacific region has by far the largest GDP, but many of its legal services markets remain underdeveloped and inaccessible to international law firms. And certain regions place greater value on legal advice than others. U.S. companies spend nearly twice as much of their revenue on legal services as the global average, according to legal research firm Acritas.

A sensible estimate is that the U.S. comprises almost 50% of the global market, with revenues near $300 billion, according to data on combined regional law firm revenues by MarketLine Industry Profiles, as well as other law firm research. Europe, including the U.K., comprises just over 25%, roughly $170 billion.

There is less comparable data for the Asia-Pacific region, but based on regional government reports it probably makes up about 12% to 15% of global market revenues with a share around $80 billion. That leaves a remaining 10% to 15% of the market to all other regions, such as Latin America, the Middle East, Africa and Canada.

The numbers may not be perfect, but they offer a good starting point. Using data from ALM Intelligence and Legal Compass, it is possible to assess how firms have distributed their lawyers globally and how well they have allocated resources on the basis of relative market size.

Unsurprisingly, U.K. firms are too small in the U.S. to be considered truly global. Only a few have more than 15% of their lawyers based in the world's largest market, including trans-Atlantic firms Bryan Cave Leighton Paisner, DLA Piper, Hogan Lovells and Norton Rose Fulbright. On a market-size basis, they are too big in the U.K. Similarly, the majority of U.S.-headquartered firms are too focused on the U.S.

Even firms that are well known for being international, such as Baker McKenzie, White & Case and Cleary Gottlieb Steen & Hamilton, tend to be either too small in the U.S. or too big in Europe—or both.

Other firms get a lot closer to achieving optimal global distribution. Weil, Gotshal & Manges and Latham & Watkins, for example, both host about two-thirds of their lawyers in the U.S. and about 30% in Europe, but only 5% or less in Asia.

Closer still are Jones Day and K&L Gates, both of which count slightly less than two-thirds of their lawyers in the U.S. and about 20% in Europe. Jones Day is slightly undersized in Asia, while K&L Gates is slightly big there.

But there are two firms that stand out for closely reflecting the size of the various markets. The first is Shearman & Sterling, which has just over half its lawyers based in the U.S., a third in Europe and 10% in Asia. Mayer Brown, meanwhile, has just over half its lawyers in the U.S., about a quarter in Europe and almost 15% in Asia—a nearly perfect match for the size of each market.

None of the geographical breakdowns really matter if firms are not successful with clients in each region, of course. Firms can do fine in just one region as long as they are bringing in mandates, especially if that region is the United States, the most profitable market, as some of the Wall Street elite would attest. It is also worth remembering that this is just where the markets are at the moment, and they will change. Asia has the biggest growth potential, especially if the Chinese and Indian markets are liberalized.

But if you were to design a global law firm from scratch, these regional breakdowns would certainly be factored in. The geographical balance of firms like Shearman and Mayer Brown suggests they are well-positioned.

James Willer and Anna Zhang contributed to this article.