The frames for LRS (long range system) do not differ so much from a classic quadcopter frames, but ocassionally there may be a need for special positioning of the antennas, battery, GPS and so on so the frame sometimes needs to get customized.
Nowadays, a 250-sized frame can be satisfactory for long range flight, but still a 400-450 size frame is recommended, mostly because of there is more space for positioning the electronic devices. From the 450 size category, kind of non-symmetric frames are rather recommended, that have enough space for mounting all he electronic, enough space for camera mounting (in the nose, while the props do not hang into the camera picture) and enough M2M for using variable size motors and propellers. The cheapest of these non-symmetric frames are referred to as ‘reptile frame’, and can be found at meny providers.
Even, the bigger the frame, the easier is to find it in case of unplanned crash situation. This is especially true for long range flight.
Having a large enough (capacity) battery bank is a must as well, so it have to be considered as well when choosing the frame.