すみません、体を洗うための小さいタオルはありますか？ — Excuse me, do you have a small towel for washing my body? (Onsen · Sento, Formal — to staff, JLPT N4)

Gesture & etiquette

Point to yourself and make a washing motion, or slightly bow when asking.

Don't bring your large bath towel into the bathing area. Use the small towel to scrub your body *before* entering the bath, and don't use it to cover yourself *while* in the bath.