For flights under 5000 ft, in a 2 - 4 lb 2.6" dia. rocket, I would stick with 29mm, if the rocket is 3" dia. and in the 4 - 6 lb range, 38mm would work pretty well. If the rocket is 8 - 12 lbs I would definitely go with 54mm. When you get to a field with a higher waiver, put the large 54mm in it and push for 10,000 ft. In the mean time, adapt down to 38mm and have fun.
I have rockets that are 54mm that i have never flown with actual 54mm motors.
Having been around research motor makers, you always hear a term, "the smallest motor i will make is...?", and it seems that the more motors the person has made the larger that number becomes. 1 guy actually told me a 75mm is the smallest motor he will make. You would think that might come from arrogance, but in effect it's actually becuase of efficiency in scale. I make lots of 38mm in pourable, and its as hard as a 15lb M motor. Just eventually, you quit working hard to accommodate for less.
I was once a proponent of put a big hole in it and adapt down.... but not so much any more. it's work. repeated work. work that will never end.
For me 38mm is the ultimate high-power motor. from G al the way up to the K... such a broad swath of motors. rocket - motor combinations are the best youll see.......
54mm is a very Niche diameter, here the biggest benefit you'll find, is it opens up to moon burners and non-typical dual thrust motors, or odd grain geometries, you dont see in 38mm(or above I impulse because of L/d ratios), but in a smaller than 75mm or 98mm price point. this doesn't exactly drive my go, so it was logical for me that i went directly to 75mm after that. the 38 stops where the 75 begins for standard bates grain type motors.
I have a 12 lb rocket that i can fly on an M/6000ns motor, or a J 800ns motor.... but, i could even put in a 75mm k. without additional work.
I am not a rich affluent guy, so i cant buy it all... So I went and skipped 54mm investment altogether. I fly mainly loki 38/740 38/1200 76/8000 98/12500, and have a 75/4000 mostly missiles case.
as you can see, all i would need is one 75mm x 2000ns case... this would let me play the entire range of impulse with out a lot of hardware.
So, i have 38 and 75, i never adapt down because.... its work, and there's usually a motor that will drive the way it needs to. save the oddball 54 rockets for 1 off projects of moonburners or dual thrust altitude attempts...