Are you doing it between the studs, or cutting into studs and adding cross-bracing? You don't mention the height of your ceiling. Making a paper template would be a good idea. Generally, those niches in a normal height wall would be a shelf at the level of a table, and the top of the arch even with the top of close-by doors. However, it really needs to be eyeballed.

I wonder if the niche will be purely decorative. In new construction the spacing between studs is generally 26" on center. I'm not sure what you plan to use to face the interior of your niche but I would maximize the width. Doing this a nice height for the niche would be 28-32". Hope this helps.