Hoshino Resorts KAI Kinugawa
星野リゾート 界 鬼怒川
The rule is Hoshino Resorts' group-wide FAQ. It says the group doesn't restrict guests with tattoos, out of respect for other people's cultures, and that any refusal is at the ryokan's discretion rather than by size.
私たちは、お客様の文化背景や価値観を尊重し、刺青・タトゥーをされているお客様の利用を制限しておりません。しかしながら、すべてのお客様が快適にお過ごしいただけるよう、他のお客様が恐怖心を抱く可能性があると私たちが判断した場合、温泉・大浴場のご利用をお断りする場合がございます。
Best for: couples arriving on the direct train who want a Hoshino Resorts ryokan in the valley
The same FAQ sets a different rule for baths that non-staying guests can also use. There, a tattoo must be coverable by eight seals of 8 cm × 10 cm (3.1 × 3.9 in). This property's baths are for guests of the ryokan, so the unrestricted half is the half that applies.