3x12,,,that size will be perfect. so figure out where you want the niche placed. normally it will go in between the studs. If you want a custom size, the studs will have to redone:not a big deal. just plan ahead. The best advice I can give you is to make sure your tile setter knows his stuff. do you homework on him. check his license. ask for references. know how the shower is going to be waterproofed. you can google all of this yourself or go to JohnBridge Tile forum and read up for a few hours. The best way to avoid costly mistakes (and believe me I say we have seen some unreal showers that tile guys think is perfectly fine!) is to know how it's supposed to be done. Sad to say in this day, you have to be able to check the person, the expert, that you are hiring. So, just know what needs to be done and make sure your tile guy is going to do it.
Waterproofing is number one on the list before tile goes up. When your current shower is removed, it will look like this underneath (above). If he's putting up cement board, there has to be a moisture barrier behind it just in case water gets behind those tiles. That cement board seams all have to be taped and mudded and should have a waterproof membrane applied. (no green board in the shower). The shower pan needs to be waterproofed and sloped properly. This is what you need to know so you can make sure your tile guy knows! Jjust do your research on "how to tile a shower".
everything you need to know about tiling and prepping is here.
http://www.johnbridge.com
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