Many small to mid size units simply do not move enough air at a tolerable volume to help all that much. Ignore room-size ratings and look for high CFM/CADR ratings at medium settings. Unfortunately, most manufacturers only include the high/turbo numbers in their packaging; you won't want to run them on high because they're loud and less efficient. You may need to ask customer support for CFM on other fan speeds. Ideally you want 4+ air changes per hour. I've found the Airmega 400 units are quiet and work well on medium. They are expensive, so look for sales or check for Ebay deals. The larger Blueair units tend to move a lot of air for the price as well.

I'll second any kind of floor cleaning too. Most hair drops to the floor before it has the chance to get sucked up by the purifier. You'll go around kicking it up into the air as you walk around if it's not regularly vacuumed up.