Why_wolf's case suggestions are good for people who want a full sized gpu but you can find much smaller since you don't need one. If it's a thin mitx mobo, there are thin mitx cases like the silverstone petite series. For a regular mitx, mx500, m350, Antec ISK 300-150, also look at in win bp and bq series.
Mitx is not the smallest. Although it's easier to find barebones and prebuilt mini pcs, the common smallest form factor is mini stx.