8" space is good enough for small sconces. Somewhere between 4"-6" would fit well and brighten a master bathroom. I have attached 2 products for your consideration. Hope this helps.
[6" sconce with shade[(https://www.houzz.com/products/1-light-chrome-finish-6-wall-sconce-light-with-white-shade-prvw-vr~65504327?lid=49531384)

[6" sconce full crystal[(https://www.houzz.com/products/2-lights-chrome-finish-6-crystal-wall-sconce-light-square-shape-prvw-vr~66567007?lid=50495821)

There really only two rules:

1) What's safe

2) What looks good

You can use very little of the space or fill it up entirely. Just depends on the fixture and the design.

Not trying to be a smart ass. But a picture of what you have instead of what someone else has would be more helpful in getting good help. Or not.

Joe