Game assets for oblique projection games.

Hi everybody I'm planning to sell a platformer tileset on the Asset store soon. These graphics are based on popular 2.5D games with oblique projection feature (simulating a sense of depth). Here are some examples, Prince of Persia, Battleblock Theater & Contra 2.

My question is:

In order to place your character in the "middle" of the block, how do you set up the collision area?

And Could you please tell me which are the needed characteristics of the sprites in order to improve or make easier the level design? that is to say how the sprites must be delivered to make easier to use them into the design of levels.

I really need your help in order to develop an useful and versatile game asset pack.

Any comment is welcome. Thanks in advance.