How to edit an imported .OBJ or .FBX Textured Mesh File

I have imported a small set of textured mesh in .obj format that is made up of several components (lot of geometry). I can right click and edit large component in order to select the smaller random components and then delete. I would like to know how to cut a uniform square area out of the mesh. Here is a screenshot of what the import looks like with one of the non-uniform components deleted. Please let me know what tools will allow me to complete this operation. Thanks.