I would go as large as you can afford as larger tanks are more stable than smaller tanks, this gives you some room for the errors you will inevitably make (we all do/did/will). As others have said, you're ahead of the pack by doing some reasearch first.
Another thing to consider is placement of the tank, not only ion realation to windows, walls, heat/cooling vents and such, but alos in relation to your floor joists. It's better, if possible, to place the tank perpendicuar to your joists so you have more than one helping hold that weight. Course, if your house is built on a slab that point is moot.
Another thing to consider is placement of the tank, not only ion realation to windows, walls, heat/cooling vents and such, but alos in relation to your floor joists. It's better, if possible, to place the tank perpendicuar to your joists so you have more than one helping hold that weight. Course, if your house is built on a slab that point is moot.