I've made a "friendship" quilt several times, and used this pattern (I hope the pic comes in).
One I did for my son-in-law, a lap-sized quilt for him to carry to his chemo treatments. His mom helped me get a BUNCH of signatures. We handed out blocks (secretly) and had family and friends sign, with their comments for good wishes, prayers, etc. Later, I joined the blocks together and quilted it.
Both my daughter and son-in-law were in tears when we presented it to them, and it is one of their most prized possessions. (BTW, he's been cancer-free for 4 years now).
This block can be very easily resized. I think you need to consider how many people may want to sign, so you can determine how many blocks, and then determine what size the blocks then need to be.
We used several different colors of the Pigma pens.
Oh, and I meant to say that I "reversed" the light/dark settings in this block, so this gives them a nice white area in the center to sign.