how big should my tank be?

[Chicklet]

I've had alot of betta's over time in small bowls all the way up to large Community tanks,
Since then My bowls have basically been trashed, 1 gallons & two gallons are only for Emergencies,
3 to 10 gallons hold mostly Betta's, (althought I have 3 gallon tanks here I would never purchased another).

Overall I have found the 5 gallon glass (cycled) the best for a lone betta,
They don't take up to much room and the betta will be comfy with just a partial weekly water change.

If your home doesn't maintain a good temperature or tends to fluctuate then you WILL NEED a heater..

[xxabc]

If I got the bookshelf from Petco, I'd either only divide it in half, or keep it to a lone betta. I know they love their swimming space (horizontally, not vertically!) , and IMO, bookshelfs look beautiful when decorated properly.
TeenyTinyTofu, that Walmart 5gallon starter seems to be a heavily popular one. I have one too, and they're always selling out at my Walmart.