Are you talking about the bow size or the band size? if you mean the band size I guess I got 3/8 for Rio(I order them in packs of 500 so they last awhile) and he too has alot of hair. I bought bigger ones once and it was just too many times wrapping around for me. If you are talking about bow size that is dependant on the size of your dog. Since you didn't mention 7/8 tho I guess you were strictly referencing the latex bands.
