Small ones I don't mind (money spiders, and generally house spiders, long as I don't have to go near them)
Mid-sized ones I tend to avoid. These are the particularly large house spiders, anything with weird black/yellow/white/red markings and such. I won't go do far as to run away screaming, but if I see one running along the floor near me, it's a very quick "quick-pick-up-legs" until either it leaves sight or doesn't move for long either for me to make a break for it and slip away.
Large ones, like tarantulas and such, I won't even go hear unless they're behind something like glass (in a tank or whatever). And I don't care if your pet has been de-fanged and/or had its poison sacs removed, there is no F'ing way I'm getting close to those buggers!
If you want to know my irrational fear... it's bees and wasps, and hornets...
At home or such, I will liturally scoot away and even scramble to get out of a room if I see one, even to the point of squeaking loudly if it comes near me. If I'm at work, though my heart may be racing I tend to keep control of myself if there's one about (unless it comes near me, at which point I tend to move rather fast and flail)
As for why I'm this way I think it's because of the many times when, as a child with glasses, I had the damned things fly between my glasses and my eyes. Not a pleasent experience and I've had it happen multiple times in my life >.<
