If you just purchased your trailer you might want to check your axle
. Most if not all bigfoot
17's left the factory with a 4 inch drop axle
and that puts the trailer lower than many of us like. Your trailer may have had an axle
modification. The original owner might have changed the drop axle for a straight axle or might have had the original axle remounted under the springs. either modification would give you lots more tire and wheel options. Since you said you will be on dirt and gravel roads you should consider an axle change along with new tires.
I put a straight axle on my Bigfoot
and went to 15 inch wheels and tires. I picked up 5 inches of ground clearance and am very happy with this change.