We currently have a 18 foot hybrid trailer that up to six weeks ago we will towing with a 2007 Dodge Nitro. We upgraded to our 2009 FX4 six weeks ago. The Ford didn't even know the trailer was back there.
A week after purchasing the truck we upgraded our trailer to a full travel trailer. 27 feet, GVWR is 9,000 lbs. The max for our truck is just around that. The trailer's dry weight is less then 7,000 lbs and since it's just my husband and I there is no way we will fill it with 2,000 of stuff (although it is totally possible).
You can get a 30 plus foot trailer but you will need to get a light weight trailer. KZ makes many off great low weight trailers.
If you have questions, try going to www.rv.net
(I am a memeber of both forums) these people know everything there is to know about towing with F-150s. Any question you have they will be able to answer it.