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.
You can also take to fondant smoothers and smooth them towards each other on the corners and edges if you want to sharpen them up some.
The best advice I can give is to make sure your cake is VERY cold. Put it in the fridge so the buttercream is nice and firm, and don't take it out until your fondant is rolled and ready to lay over the cake. You'll get nice sharp, square edges/corners every time!