You really have to find out what kind it is first. There are all different types of day geckos, and some stay fairly small, while others (like my Madagascar Giant Day Gecko) get very large. Some species can be housed in a community, others will attack tankmates. It really makes me angry that Petco would even consider selling them without even saying what type they are, and letting people know their requirements!!! I know we are talking Petco here, but there has to be a limit to even their irresponsibility!!
Can you describe what the geckos you are seeing look like? Colors and size, and distinguishing features? RD Bartlett has a good book out about Day Geckos which has full-color photos of many of the types, as well as their care requirements. Amazon.com usually stocks it, or you may even find it in the library. Just make sure you know what you are getting BEFORE buying, or you could end up with a real mess!
Wendy