I'm Pooh sized, but I just buy whatever fanny pack that I think i'll like (my most recent one is a Disney pack).
Then I extend the belt of the pack - you can buy the nylon webbing at any local fabric/craft store - it needs to have SEWING fabric, not just quilting fabric. I get mine at Joann's.
Then you simply cut the buckle off one end (figure out which end you want to do this on!!!), sew the new belt fabric to the existing, either by hand or by machine, and attach the buckle by sewing it back on.
This does NOT have to be pretty - the first time I tried this, I used fishing line to do the sewing, and did it by hand. Took maybe 15 minutes, and I was able to use the fanny pack I really wanted instead of paying extra for a plus-sized pack, assuming I could have found one at that time!
Total cost for the webbing was under a dollar.