I do a fair amount of pedi dentals. I start with the same size ETT as I would try orally, and with lube and vasoconstrictor it usually passes. The issues are length of ETT and leak. Smaller ones are shorter and sometimes don't reach as far as you would like. And there is often a large leak with smaller ETTs, though if you keep spontaneous ventilation it's not such a big issue.
It also depends on the size of the kid...If they're smaller than usual for their age, might have to go down a size.
Don't force it, it's a pain to have nasal bleeding. If it won't pass, go down a half size.