If you think size matters, visit Dr. Stein and get some Extendze! But if you are worried about the charcoal basket for a UDS-size dosen't matter! For me its all about the duration of the cook.
Use what you got! 18" - 22" charcoal grates, or an 18" cook grate or a combination, I've tried them all with success! I'm cheap too!
My preferred baskets are ones that use a 22" cook grate and an 18" charcoal grate wired together in a cross (#) pattern, this allows me to use lump without the duff falling through the grate ( I use 2 # grates in my kettles also)
This size easily holds more than the 12 lbs I will use for long cooks, and most times the meats are done long before the coals are out!
For those short cooks @ 4-6 hours, I made some rings out of expando that are about the same size as a Weber chimney, and they hold @5-6lbs of briq
and just set them inside the baskets-these are the ones I use the most when I cook using the drums.
The thing about the drums is they aren't fuel hogs, and when I shut mine down after cooking I can usually reuse some of the charcoal the next time I cook.