Market cap is not an indication of size at all. It's an indication of, well, present market value.
For size, there are few things you can look at it: there is size based on net worth, total number of employees, total sales in dollars, total unit sales, etc. Most of these are only useful when comparing to companies within the same industry, however. In fact, most useful statistics are really only useful when comparing companies within the same industry. That's why so many people focus on market capitalization, because that is comparable across industries.
Biggest market cap, though, as you point out, doesn't mean that much. A more useful statistic for investors is the earning per share (EPS), which gives you an idea of the economic viability of a company .