some multiple of a 30" row, and most likely a 4,6,8, 12, 16, 24, 32, 48 etc, at least it was
so one spraying could follow the rows in row crop, usually the nozzle spacing was such to spray between the rows,
my old sprayer booms were based on a 6 row unit, as I had a 6 row planter and header,
now the big stuff uses GPS and 70 to 130 or more foot booms, the engineering behind the booms is amazing, to get them to ride at 15 mph through a field and float and not crash on the tips, and to fold up, etc,
probably for your needs about 20 to 21 foot booms would be good,