If you go to the POR info thread, there are some pics of a larger child laying on it. http://www.disboards.com/showthread.php?t=2904701
You can also google it and look at images.
I'm thinking of taking a measuring tape and taking pics to answer this question in the future!
Unfortunately, Disney haven't updated their official website. Here is what they said about the trundle bed.
"Some rooms offer a trundle bed (Trundle bed is 35" X 65" and not recommended for Guests over 9 years of age)."
I'm assuming that the Murphy bed also sleeps a child 9 and under I don't think my daughter, 13, will be able to sleep on this one. But my 5 yr old should have no problem with it.