How fast can you and how fast should you are 2 totally different things and depend on many different factors such as trailer type, size, and weight, road conditions, tow vehicle conditions, brakes on trailer etc.
60-65 is the normal safe tow speed for an average trailer but laws in some states restrict it to 55mph.
Which gear you tow in depends on the trailer as well. A small (low wind resistance) or light trailer you can get away with 5th (OD). Anything heavy or large you should keep it in 4th (on a 5spd). If you are not sure you can try 5th and see if it can hold speed with ease or not. If it constantly shifts up and down you should leave it in 4th.