I think 100 vs 1000 should be a valid fight. There may be a place to draw a line, but not 1:10.
Maybe 1:100. I haven't seen anyone do it yet but it's just a matter of time before someone realizes that they can stall a huge army forever just by attacking it over and over and over with 1 man armies. oh noes, what have i done
I do think that a size 0 force should auto-lose though. It shouldn't even schedule the battle.
Ditto for armies with no weapons, unless both sides are weaponless.
Really there's only so much amusement to be had out of having to show up for a 5am battle that the enemy has absolutely no chance at because they had 0 troops and no weapons. Shouldn't even make us log in. (And you can never be sure when a battle is going to end up scheduled for 5am because you can't be sure what their nighttime is. You attack at 9:30pm and, welp, 5am it is.)