Hi Skychief,
Your question cannot be answered because you haven't mentioned what kind of rifle you want to build. Do you want a "longrifle"? If so use a long barrel and accept the weight. There were many fullstocked flint rifles that had very short barrels. European gunsmiths made them all the time. Even American longrifle makers sometimes used barrels <42 inches. I just did a quick scan and found 15 rifles featured in RCA volume 1 with barrels >37 inches and <42 inches. My advice is to make the gun with a barrel length that works for you and if you are a gunsmith with at least a modicum of design ability, you will be able to scale the dimensions to make the gun look slim, long, and pleasing. It takes tremendous knowledge, experience, attention to detail, and skill to produce a "bench" copy of an old gun (note I don't write "original" gun because all guns made by members of this forum are originals in some respect). However, in my opinion, the test of a good gunsmith is defining function and then fitting a form that knocks your socks off.
dave
dave