I have a 40gal breeder and just measured it. The inner dimensions are 35X17. If I understand you correctly, you are wanting a stone piece to lay on the bottom? You may have to cut 2 separate pieces as laying one giant slab of stone down onto the bottom would be difficult, as you won't be able to have your hand under it to support it. It may be easier to use many smaller pieces of flat slate and make a "puzzle" of sorts fitting them like a mason would do with stone. Whatever you do be careful, as using large stone to lay down onto glass can quickly ruin a nice tank. Why the stone, are you going for a certain bio-type or something? Are you using substrate on top of the stone?