Unit scale is left undefined in A&A. It seems to shift as the game progresses, as armies expand in the game.
Another issue is quality. The large German starting force represents their superior quality, so German scale is somewhat different than the other nations.
If you are designing a scenario based upon some sort of OOB, the total manpower in an army is probably a better measure of its size than a count of equipment. If you are using a wargame as a model, you should translate units specifically from one game to another. If you are using equipment counts don't forget unit size considerations. For example, British carriers were much smaller than American & Japanese carriers, while Russian Armies were comparable to other nations' Corps.
If you must have numbers, the starting point is John Ellis, World War Two, A Statistical Survey 978-0816029716
A posting by Imperious Leader has the following numbers
Land: 3-4 Infantry divisions = 1 unit ( Good quality) 5 infantry divisions = 1 unit ( average quality) 6 infantry divisions = 1 unit (poor quality) 2 Armor/Mech Divisions = 1 unit ( good quality) 3 Armor/Mech divisions = 1 unit ( average quality) 4 Armor/Mech divisions = 1 unit ( poor quality) Artillery (subjective based on level of artillery in infantry and other factors but generally 2,000 pieces gets 1 unit)
Sea: 3 Battleships = 1 unit 4 Carriers = 1 unit 10 Cruisers = 1 unit 20 Destroyers = 1 unit 30 submarines = 1 unit 2,000,000 Gross Tons of Merchant Shipping ( rounded up) = 1 Unit
Air: 200-250 Front line Fighters = 1 unit 200-250 Front line Fighter- Bombers= 1 unit 450-500 Front line Bombers= 1 unit