The most important part if you plan to hunt with the rifle is the LOP. What you can easily shoot with a T-Shirt on and what can easily be shot with thick winter clothes on is not always the same. Most of the rifles built from kits are all 14" LOP. For a lot of people that works well, for others it doesn't. A quite tall National ML champion in flintlocks shoots rather short LOP for his height. I chose the wrong LOP with my first custom rifle, because I measured another rifle in summer with a T-Shirt on. This rifle did not work out hunting for me. Rifle is a tack driver on the bench and for summer shooting, but in cold weather hunting clothes, my shoulder moves the gun just when I reach for the trigger. Many misses. $2k lesson learned. Corrected this mistake and went 3/4" shorter and no problems hunting and target shooting in all weather conditions. IMHO this is one of the great overlooked measurements for flint guns especially.
Drop at heel of the gun and cast off: When you shoulder the gun, your eyes should automatically align with the sights.
Body height, arm length, body build is different with everyone out there. Make sure to know if you are the average guy or have certain issues.
Do you have issues carrying weight? Get the largest caliber the barrel weight offers. Get a swamped barrel or octagon to round for good balance.
Do you have to walk a good distance to your hunting spot? Would a gun with a sling be handy for your situation?
Longrifles do not carry good on slings. Jaeger rifles do. Shorter barrels, large calibers, little recoil due to wide (early) butt plates.
Later longrifles with smaller calibers and hawkens have more crescent butt plates. That works ok standing or kneeling, but not for prone shooting. In the developement of the American Longrifle, versatility of use was imho left behind somewhat.
With Jaeger rifles and flintlocks, stay away from Italian reproductions. The locks are flint eaters and the guns do not have flint breeches. I started out with a Pedersoli and it nearly made me quit the sport. Gun was redone by me in a Pedersoli to Lancaster Conversion. New breech, new lock. Now good gun. Never worth the trouble saving some $$ on Italian flint guns. Exception: Percussion and the Brown Bess (has real flint breech).
And I shoot an original Jaeger rifle from around 1745.