POR trundle bed size: Our 5 year old son slept on it last year. He had no trouble, but didn't like being so low to the floor. The beds are higher than usual and so the trundle really seems low, you are looking under the beds.
I would say as long as your daughter is not taller than 5'4" she will fit ok.
The rooms are really small, the trundle opens up between the double bed and the door to the room. We were packed in there like sardines! My two girls 7&9 took pity on him and let him sleep in with them for the last half of the trip. It was close but at least he could watch the TV then.
As far as the hilton goes, I would seriously consider it. We didn't stay there, but stayed at the Best Western a few doors up. We got a room with a Disney view, we could see the fireworks most nights and we had a balcony. The rooms were a little older, but considerably larger than POR. Now, you don't get the same feeling as being "on property" but it's pretty close. There's a disney shop in the lobby and a few "Mickeys" here and there.
If my hubby and I hadn't bought into DVC on that trip, I had plans to scout out the hilton as our next spot to try. I really liked being able to walk to DtD from those hotels, and the hilton is the closest. You can get to disney transportation there. Lots of restaurants etc. and you definately get that Disney feeling walking around there.
The busses at the BW were not bad, less crowded than disney busses but came less often and picked up in the middle of the parking lot. But most of the time we took a bus to DtD and just walked "home" from there.
I felt that the hilton might offer a really good option going back to WDW. They had a great location, good pool (from what I read) and they offer extra magic hours ( a mixed blessing).
I just don't know how the pricing stacks up, but maybe you can get some sort of deal on-line via priceline or some other discount source.
Good luck, hope this helps
