I have a tee-shirt quilt in the planning stages. I've researched pictures of t-shirt quilts online and I think I prefer the ones that have sashing. Without the sashing they look a bit chaotic I feel. If it is to be hung I think orienting all the patches in the same direction may make it easier for the viewers to see/read the patches. Various sized blocks would be ok and may be necessary due to sizes of patches from shirts. Overall size? what is size of wall it will hang on? other than that it could be any size you want(this may decide if you want to include sponsors from shirt backs). Only thing I can think to do with left over pieces is to cut into manageable pieces for cleaning rags.
Good luck with your project.