1) have enough room for them to sign. We're accustomed to signing on some pretty short lines on myriad forms we have to sign, but quilt blocks seem to be different. Maybe they seem more daunting, particularly to those who haven't done it before, and people are afraid they'll mess it up.
2) have "boundaries" for the signing built in. It's not reasonable to expect people to allow for a 1/4' seam, even if you ask them to, or explain why. It is much easier for everyone if the piece they are signing is already pieced, all the way around, into the block. My retirement signature quilt had a bounded piece for friends to sign, and it seemed to work perfectly.
3) be a shape as close the to a rectangle as possible. (Though, the larger an odd-shaped piece, the easier it seems to be; see #1) I've asked people to sign diamond-shapes, in a Square-in-a-Square, and it's harder to "map out" how to fit your signature into the shape. After the fact, of course, I realized that I would probably be scratching my head, myself, if faced with a small-ish diamond shape.
It wins 3-1/2 stars: the hexies are 2" on a side so they're large enough; they have boundaries built in (the pieces are whip-stitched together, so seams are already set by the basting around the paper); it isn't a rectangle but at least they aren't sharp points to work around they're big enough to mitigate that issue; and they stabilizer is also built in, in the form the paper that the fabric is basted around.
(Oh, and as a bonus, this can be another sample for the English Paper Piecing class!)
What blocks would you nominate as good signature blocks?