Those prepaid cards cost money! Either just use cash or keep the cash handy to pay off your regular credit card!
RCI uses Coke products. If you really want Pepsi, you can bring some onboard with you.
I've always found the beds to be a bit larger than queen, but not quite King. My hubby is a big guy and we're very comfortable.
I would make sure there are few sea days--stick with itineraries that have lots of ports. I love sitting by the pool and reading--but if you're not into relaxing, and don't "play well with others", then you won't want to have day after day on the ship. Perhaps a shorter cruise would be right for you--at least, as a start!