I design most everything at 10% of final size. If it's something that will be looked at from fairly close I use 720 to 1500 dpi. If it's something to be seen from pretty far (billboards, mesh banner) I usually go between 500 and 350 dpi.
When you send it to the rip you then increase it by 1000% and you get your final size and appropriate resolution.
Of course, offset is a different beast all together. That I always lay it out at 1:1 with a minimum of 450 dpi
jc