You’ll need at least a 6 x 10 inch hoop to embroider most hats. A hoop this size gives you enough space to embroider designs across the front of caps and hats without rehooping. For full coverage designs like all-over embroidery, a tubular hoop works better by wrapping around the crown.
First, measure your hat and then choose a hoop 2-3 inches wider to provide full coverage.