Small spiders seem to be ok as well. I've fed my VFTs several small spiders and they've done just fine. The hard part is getting the spider in the trap and making it stay there until it triggers the hairs. I usually just leave the spider on it's web and bring the VFT to it.. it's much easier to get the trap around the spider when it's on the web..
I just try not to do this outdoors or the neighbors reaally look at me oddly..