We had a pretty large group 20-25 people?) and all fit inside and had enough room to swim around. It is pretty dark and you do enter through an opening in the rocks and go through a little corridor that seemed to bother some a little but this is pretty brief. You can watch other people go in and decide not to do that part or if you get inside and find it isn’t for you the guides are very accommodating and will show you the way out. Have fun!
Get an answer from The Challenge Mallorca staff and past guests.