Great question. We bought our first TT and didn't consider this. 2 years later(this year) we've upgraded and still didn't think about it until we got it home.
Let me tell ya... wowsa. We bought a gulfstream Amerilite 259BH model (just shy of 29 feet ball to bumper). With the slide in we can still access the fridge, and walk all the way back to the bathroom, and open/close the door!
I'm not a thin guy, and it's no issue. In fact we could camp this way. The slide gives us a ton of open floor space, so with it in we just have an aisle like in a non-slide. On our slide is a sofa and dinette.