How strict are the tattoo rules in onsens?

I am traveling to Japan with my girlfriend I am not inked and she has a small 5-6 cm butterfly below her rib cage.
I am currently in search for an onsen that fits in our travel route and was wondering how strict the tatto rules are.
Do they differentiate between sleeves and giant tattoos and only one small one as described or is any kind and size of tattoo already to much?
Thanks in advance knowing I could also search for a normal onsens would make things much more easy