I went with an 8" but wanted a 10". Looked @ the weight of the 10 and took the 8! BUT by the time you try to clamp some things to an 8" the means of clamping may take up a lot of the real estate. Next step make a 11" aluminum plate to extend the area. Works but need to be careful tightening the hold downs. Then I wanted to put a chuck on. Got a nice 8" chuck but by the time the chuck is mounted the assembly is too heavy for an old man. So I have to take the chuck off each time PIA! It is a very nice chuck that I machined a back plat to fit the D1-4 spindle on the lathe. I got an excellent very "snug", read a bit too tight, fit so it's another PIA to change. Can't win! Much depends on your use as to size but for me I wouldn't like to have less than an 8". I've used the indexing plates a few times. Can be a head scratcher to learn to use. Great for cutting gears though. Put blocks on the bottom of the RT that precisely fit the T slots of the mill. Makes setup easier. When setting up vertically you can run in to the problem of stick-out and mill spindle/tooling limiting how close to the RT you can machine. Turns out a tailstock is nearly always required.
YEA OL' Rabbet hole, again!