Although I haven't had the time lately to look at them all in earnest, the Rimsky-Korsakov on-line orchestration lessons are just magnificent in the way they are being presented. Even if you've been through the actual text or something similar, I think taking even just a cursory look at some of these would be well worth the effort for anyone. (I'll be taking a look at lesson 10 today!)
Here are a couple of very short pieces ("orchestral vignettes"?) I just quickly sketched out for practice with GPO to utilize some of the orchestration principles from lessons 6 & 7 (originally posted in the RK section by GG).
Why not?! A symphony can take any size, shape or dimension, from Leopold Mozart's Toy Symphony to Mahler's 8th. I'm certainly looking at doing one. Give it a try. You are definately developing the necessary tools.