The studs in the wall is what limits you. Studs in general are 2"x4" deep The niche fits within that space. studs are normally 12"-18"s apart and that limits the width. So when you open up the wall you can see just how big it can be.

I agree, the size you've indicated would be the maximum niche one can get without having to alter the wall studs behind.

I usually use the gauge of my client's biggest bottle of shampoo - if it fits, then we're okay!

Hi princess_q,

If you're looking for an easy way to increase the depth of the niche without altering the wall framing, you may want to consider a making the perimeter and shelves of the niche from stone or quartz instead. This gives you the opportunity to increase the depth of the niche by around another 1" more than if it were integrated tile, as the surrounding wall tile can terminate into the side of it. (Shown in the first photo below)

For a coordinating look and cost-effective approach, you can use the same stone that you have for your countertops, and also use it for a shower bench, and curb etc. An alternate approach is simply to increase the height to accomodate additional shelves. (The second photo was installed in a 2x6 wall)
-Steve