As far as how much supply each unit consumes...I'm not totally sure.
When I say "Transport Capacity"...I mean how much of your transport infrastructure that isn't being used to move units...
What you don't use to move actual units, is abstracted and used to move supply from your production centers, supply depots etc...to the front line areas or wherever else needs it.
Transport Capacity = Available Rail, River, and Ocean transport "points". The available "points" can be found in the top center of your screen, just to the left of the main "Resources Display" (or whatever you want to call that

).
The Transport Capacity display has three columns and four rows with check marks in one of the several columns. Depending on the number of transport "points" you have used (noted in the far right of the display in each row)...check marks will be in one of the columns marked "1/3, 2/3, 3/3".
You'll see little graphics on the left of the display in each of the rows. One graphic looks like a train, another looks like a river boat...the third an ocean ship.
The train signifies rail capacity...the river transport, river capacity...etc...
1/3 = A transport capacity of 1
2/3 = A transport capacity of 2
3/3 = A transport capacity of 3
I hope that helps.