onsenjapan.net claims that タオル is used for "towel", presumably the large one. When I asked a staff member at Odeo onsen "これ は 何 ですか?" while indicating the large towel I was holding, the staff member said "タオル".
I'm not 100% convinced, though. I half suspect that the glossary (and the staff member) may have chosen a gairaigo term over more traditional terms since it'd be easier for foreigners to remember. Also, in English the terms "large towel" and "small towel" feel very clumsy to me, so I would have thought that Japanese would have kept its existing words for the large towel and small towel.
I tried seeing if towel was mentioned in http://ja.wikipedia.org/wiki/温泉 , but unlike the English-language version it didn't (obviously only foreigners need to google how to use an onsen!). Other pages I tried looking at were this, this, this, and this - the fact that towels don't appear in those glossaries would be consistent with it being plain old boring gairaigo, and that Japanese doesn't have special onsen-related terminology for towels.
Is タオル used for the towels used at onsen, both the large towel and the small one, or are other words used as well?