As the title says, looking for advice on target size for shooting milsurp primarily. M33 ball, distances from 100 to 600 yards. 

I'm leaning towards targets 2-3 MOA. Probaby 3MOA at sub 300 yards, 2MOA further out, but looking for advice from more experienced shooters as to what you prefer. I figure right about the same size as the rifle and ammo is capable of grouping should be right. When I don't do my part, I'll miss, when I do my part, I'll hit. :D

Paper, that's fine.

Steel targets should be much bigger. 50cal tends to knock the smaller ones around.. I.e. a 4" steel target would shortly be ripped from any mounting you have it on.

I have "the last stand" target stand system and a 20x20 steel plate. I just paint the steel black, put an empty tuna can in center and spray white over the black. That gives me an aiming point

Thanks for the advice, gents. 

For use on steel targets, use a 3 MOA size for the range you decide to shoot.

Then use cardboard cut-outs to paint: 3-MOA white, 2-MOA orange, 1=MOA black, and a sighting square in center which could be 1.5" white square.

Be sure your mounting system is stout and designed to hold behind the top edge so bullets are splashed downward into ground.

DO NOT mount solidly on both side as jacket could be sent back at you - videos on line about this (but not at 1000 yards).

Be SAFE , have FUN.

Be sure to use proper alloy of steel (AR500) and proper thickness (1"+) to give it a chance to survive.

A target I donated, to a 50BMG Sanctioned range where we shoot FCSA matches, was 3/4" thick 15" diameter AR500 mounted from back to deflect hits into the ground. When placed at 1100 yards for Match sight-in, a good solid hit by the 800+ grain solids we shoot will really toss the target around resulting in the need for a solid mounting system to hang the targets from. Our loadings result in excess of 14,000 ft lbs of muzzle energy. AP, API, and APIT projectiles are not allowed and you will be told to leave the range if you shoot one.

Highly recommend targets and mounting systems from: ShootSteel.com

https://youtu.be/jjSat8xeMws

The next links was using my rifle (but not me) shooting surplus 660 grain ball ammo.
https://www.youtube.com/watch?v=5ePPz2iT9XE
https://youtu.be/I0Net26FUAk

https://www.youtube.com/watch?v=I0Net26FUAk

Cool, that matches well with my research and results as well. using AMEX 750 factory loads, 1" AR500 holds up fine at 100 yards, which is as close as I was willing to test. 

I use 1/2" AR500 targets. Most of them are 18"x36" and one is 35"x36". The big one is very heavy too. I hang mine with chains.