Axel size difference...

[EFG]

My question is basicly: Should my back axel be wider than the front or does it not matter?

[WalkTogether]

Wider in back makes more sense.

[SteveC]

The wider the axel, the larger the distance the wheels have to travel in order to reach a desired turn angle. This is a classic trade off, stability for response time.

Wide trucks have the advantage of decreasing how much angle is inflicted on your line by surface bumps, and are more forgiving of footing errors

Narrow trucks respond quicker to weight shift. and are more picky

There are leverage differences as well, once again wider means the deck has less leverage over the hanger, this can be equaled out somewhat by running a wider deck, or higher risers.
this is one of the reasons behind the classic pintail shape, it helps stabilise the back truck.

[bean-a-reano]

A little wider rear helps with the tracking, lowing down the turning of the rear wheels. Not a bad thing in slalom, just don't over do it.

[EBasil]

Remember that wider also means...wider, and it's easier to hit cones with an extended wheel. A few millimeters is all you need.