It depends on the company!! I can recommend the Serenade 2 operated by Riviera Travel. Cabins are 16 sq m and the bathrooms have both bath and full shower cubicle. We have just been on Lord Byron, again operated by Riviera, but cabins are 1 sq m less. Still quite adequate and comparable to ocean cruisers - at least the ones we like
All have double beds - or split into twins. NOT bunks. We have been on other boats with tiny cabins - the Lady Ann and Rembrandt are two with small cabins.
I would suggest a Rhine cruise first. The attraction of the one we've just done is cruising across the Main but not too many places of interest, and on the Main and upper Danube the bridges are low (and locks narrow) so the sun deck is out of use. If you find you enjoy it, then this is a great cruise.
This is the forward sundeck on Lord Byron - it really is so low.