Are all the subs using the same model?
 

It just occured to that, after looking at the museum, it would appear that all of the VII class subs might be using the same basic fuselage model, i.e., the dimensions are the same.

According to U-Boat.net, if the dimensions there are correct, the subs in-game should be quite a bit different dimension wise to eachother.

From U-Boat.net (give or take a few inches):

Type VIIA - Length: 211ft
Type VIIB - Length: 218ft
Type VIIC - Length: 220ft

In-game, is it just me or are the Type VIIA, B, and C all the same size?

Maybe my eyes are deceiving me, but it does not look like it. There should be quite a noticeable difference between the Type VIIA when compared with the B and A.

Is there any way to check the dimensions of objects in-game?

Use the port dock for this, it could be easy measure the boat sizes using other references.

For sure dev team uses the same 3d model for it , remember that there aren´t any Ix-boat in the game , probably for no time to implement it