You can just get normal untreated lumber, which would be pine, from local hardware or farm stores. I think the minimum for a hole, pipe, etc... for adult guinea pigs is usually considered to be 4-5". If you want to make it so even fairly large adults can run in and out easily you'd be best off just leaving 1 end open or half of a long side if your making the house large and not putting in any windows so they still feel hidden. For size I'd measure your longest pig and add a few inches for minimum length and take off a coulpe inches for minimum width.