Silly little bug and I go all girly.....
In this field I think size doesn't matter much 
Some time ago (in the 80s I think) I had just been stung by one of these buggers, they are about 1" long
The jaws are obvious in this shot but they also have a sting in the arse (ants are related to wasps and these ones are more closely related than most, just wasps without wings really).
They hurt like hell and about 20 bites can kill you, I commented to a work mate "Can you imagine what can pack such a large punch for it's size?", he though for about a mS and replied "The AIDS virus".
One difference between say the scorpion and a spider is the physical design of the pointy bits, all else being equal a scorpion's sting is longer than a spider's fangs and therefore more able to penetrate clothing etc. Same with snakes, some of the most venomous only have small fangs and don't pose much of a threat to people wearing jeans and boots for example, that's little comfort to me though because I doubt I've worn long pants or even shoes more than a dozen times in the last decade, unless I'm doing some physical work (perish the thought) or on a bush walk.
Rob