I am just wondering if putting 17's on (over 15's) would lessen the gap in the wheel well or would it make it larger?

Im thinking it would make it bigger

 

It will stay around the same width...you need to get lowered before you put on some bigger rims. Please do.

 

w/o lowering the car + putting big rims on = monster truck look

 

Maximas don't have enough suspension travel to "tuck" anyway.

Well, unless you get the JIC coilovers.

 

it wouldnt change the gap. The overall size of the wheel and tire is meant to stay the same.

 

I am already lowered with progress springs..... im wondering if replacing my 15's with 17's will lessen the gap in the wheel well.. or would it not change

 

the only thing that will change the gap is the lowering springs.
When you get 17" wheels you get tires with a smaller profile so that when you add the height of the tire and wheel together it is the same size as the stock 15" + tire. Going oversize puts your speedo out and is generally not recommended.

However the bigger wheel looks better proportioned to the guard making it look like it fills it better.