Regarding trailer length, you have box length and overall length.
For box length, most makers incorporate a rough version of it into the model number. For example a Lance 1985 is roughly a 19-foot box. However not everybody does this. My North Trail 22FBS actually has a 25-foot box.
Overall (bumper-to-tongue) length is 4' to 6'+ above box length depending on design. In my case, there is a pull-out tray that can be used for bikes, generators, etc.
Box length has more impact on driving. Overall length affects what campground spaces you can fit into.