OK, I'm still in the beginning stages of this so bear with me. I've been browsing ebay and cl trying to find my first bike and I'm a bit confused about sizes. 15", 16", 17", etc. are seat tube(?) lengths, correct? 24" and 26" are wheel diameters I assume. So if a bike is listed as a 26" It may still 'fit', right? I need a 16-17" seat tube(?) length from what I've read on the interweb, I guess this is why everyone says to go to a LBS.