I start off with a square fruit cake and roll the fondant to the thickness required I check to see if it is the same thinkness on each side using a rolling pin to lift the fondant on the cake. BUT I still end up with a cake tha is not the same size on each side and rounded edges and corners. Can any one help or do I have to give up on square cakes

That's just the fondant doing what it does... it naturally rounds out over the edges of a cake, no matter what shape it is. You can just press it with a fondant smoother or with your hands and shape the corners a little until it's more square. You just have to work it a little, press it out between your hands gently until it gets to a more square-edged shape.

Here's a great video of covering a square cake. The most important thing to remember is to work the corners first.