1.- every card may have a different length, yes... some can be as large as 14" I think... but they all fit in the same PCIe slot, it is the bulk of the card that can get really long. You need to check for video card max length (this is GPU clearance) that fits in your pc case.
2.- check what model is your power supply, and whether it has the needed PCIE power connectors... they can be like this
http://www.mechbgon.com/build/8-pin_pcie.jpg
Tell us whether it is 6 pin, or 6+2 pin (like in the pic), and whether you actually have only 1 or 2 of these. When purchasing the video card, it tells you how many PCIE connectors needs, it could be 1x6pin, 2x6pin, 1x8pin etc.
so, that's 2 things you need to check before purchase... 1.- whether it...