I just ordered a new barrel (heavy) from Shilen in a 6mm cal. 1 in 10 twist, to be installed on a M700. I was considering going with a 22" length to reduced weight and improve manuverability, but am concerned this might be to short. I will be using between a 80 and 90 grain bullet on varmit size targets. Expected shooting distance will be 300 to 500 yds. What is the general opinion out there?

I'd go with at least 26" to start with,you can always shorten it.I used to have a Rem 722 in 244 with a 26" barrel.It would launch 75 gr Hornadys at 3500+ and it wasn't hard to carry around.It would literally vaporize a gopher.The 6mm case has quite a lot of capacity for a 243" bore.Just make sure you leave a long trunnion so you can have it shortened and rechambered a couple times to get a new throat.And have it throated short to begin with so you can seat near the lands and still fit the mag box.

Mine is 28" on my 6mm Rem. If you're screwing on a heavy (#7 Shilen?) tube, you've already made the sacrifice & chosen a heavier rifle.

Are you planning on 6mm Rem or 243? Take full advantage of the caliber chosen. If you stay at 22", you may as well chamber to 243. I'd finish it long and go 6mm Rem myself.

The intended use of the rifle is a very important part of the OAL issue, is this a calling rifle, one that needs to be "handy" or will you be shooting from fixed positions?
Velocity is only part of the total rifle package. I will agree that to maximize the 6MM Rem case you might need more than 22"s of barrel, but I will bet that a coyote at 300 yards won't know the difference between 22-30" and you won't either.

If you want to go with the 22" barrel, you should also look into chambering the rifle for 6mm BR because it uses about 30 grains of faster burning powder to get nearly the same velocity as the .243 Win.
The faster burning powder should burn more completely and consistently in the shorter barrel and the 6mm BR is arguably the most accurate 6mm intermediate range cartridge out there right now.