Size & location of tub access panel

I don't have one...have never seen one before...

I have a typical fiberglass one-piece tub/surround. The other side of the drain is the closet of a bedroom. So i can open up the drywall....

how big is a typical "bath tub access panel" ? where is the best location to install one ?

thanks

Since the access hole will be inside a closet, I'd consider going floor to ceiling and stud to stud as close to center of the tub as possible. This would give you lots of room to work in there if/when necessary. You could make a presentable cover for the hole with a piece of painted plywood. Obviously the floor to ceiling height is way more than you probably would ever need, but who knows what the future holds?

If you don't want to go that big, the ones in our house are about two feet tall and go from stud to stud. It provided easy access to the drain and both supply lines to the tub.
Since it's in a closet you can really do what you want. The key is that if something goes wrong you can easily access all components.