Cogito, to answer your question - if you are looking for something specific, whether it is size or colors to use/avoid, or anything else - make sure you let your group members know your requests/desires IN WRITING along with your top.
For me, I usually type up a short letter or note about what I'm planning to do with the quilt along with the colors that I would either prefer them to use or to avoid. For the one I'm in right now, for example, I asked everyone to stick with red and white and use either tone-on-tone, blender, or solid fabrics. If you want people to use a specific fabric, you might consider including some of it with the top in order to guarantee your group members are able to get their hands on some. I've been in several groups now where my teammates included about 1/2 yard for each person in the swap - if we needed more, she'd mail us more.
The group size doesn't necessarily dictate how large or small your quilt will be since there is a pretty big range of available sizes for the borders (3.5" - 6.5").
For people who are wanting very large quilts (King, Queen) the tops themselves may not return to you meeting these dimensions however you can always add additional borders to "finish" the top to your desired sizes.