The way a barrel roll was originally explained to me is to think of a loop (as in looping the loop) as a coil of wire. Hold the two ends of the wire and pull them apart and hey presto you have a barrel roll.
So you can have a small barrel or a large barrel (or anywhere in between).
Whilst a good barrel roll involves only modest amounts of "G" there is, in fact, more looping involved than rolling.
The hazards of rolling an aircraft which is not cleared for such maneuvers is that if you mess it up you might end up pulling lots of G and/or exceed limiting speeds.
Recall the story of Neil Williams (who was probably the best aerobatic pilot around at the time) who when he was a demonstration pilot on the Jetstream performed a barrel roll in cloud whilst the sales people were in the back with potential military buyers sipping their gin and tonics. Allegedly nobody even noticed that he had done so!