There is a rough scale to the units (and it is outlined in the introduction section of the rulebook). Unit scale varies based on unit quality, leadership. morale, etc. - but, each infantry unit represent between 30,000-50,000 men (2-5 divisions based on nationality and quality (about 5 Soviet divisions, but about 2 U.S. or 1939 German divisions). Each armored unit represents 300-500 tanks (again about 2-5 divisions worth).
Carriers and battleships represent between 3-5 ships. air units represent between 300-500 active combat aircraft. Cruiser units represent 9-15 light and heavy Cruisers. Destroyers and subs represent between 24-40 ships/boats.