it all depends. why don't you post a pic of what it's going to look like?

but really, for something that small, go all the way across.

you want a tiny, 1 foot mantel? that would look silly. like this one

The length would be 26", which is a little over two feet. I assumed that was too small, but don't know if 3' or 6' is appropriate.

I can't post a pic because it's for a house that hasn't been built yet.

I know, that's why I said post a pic of what it's supposed to look like. (something similar, inspiration, a drawing,,,,)

is it rustic, modern, industrial, brick,,traditional, tile,,,does it sit flush to the wall or protrude? all of those types can have diff types of mantels.