Say my longblade skill is 10, is it more likely I will hit a scrib than a kagouti? I always seem to have a much higher success rate hitting small enemies, or am I just imagining it?

Your http://www.uesp.net/wiki/Morrowind:Combat#Chance_to_Hit depends on enemie's agility. Scrib has 20 agility while Kagouti has 75 agility. So its easier to hit a scrib than a Kagouti.