You have several options available to you for this. The first and easiest would be to create a static mesh with the dimensions you need in a 3D modelling program and import that file to the editor. The second would be to use static meshes present and work around the dimensions by placing them in various angles, but you would not be able to use just one as more than likely the edges will stick out (as you mentioned), the third, and while viable is not the most desirable solution would be to create a bsp brush and edit that brush to fit the dimensions you need, then turn that bsp brush into a static mesh, adding collision, etc. Any of these can work and I would recommend playing with each to see what is most fitting for your needs. Below are a few links to help you with your decision: