Viking River cruises have Queen size "hotel type" beds in A grade cabins on their newer ships(dont know about other cabin grades or the older ships) which were v. comfortable. Hope this is of some help. Cynthia.
Never been on a river cruise but would love to go down the Rhine visiting all the vineyards hic hic but they are generally smaller than cruise ships so would not expect them to have large bed you would have to make the request if interested and see what answer the travel agent/river cruise company come back to you with.
Why don't you try the other site www.rivercruise.co.uk. You will be asking the opinions of a more river cruise based group of travellers.