seat post?
 

looking at seat posts on ebay. i don't know what size i need, since i have always bought them by trying them at the store. i need a long one because i ride a 56cm road frame, while my inseam is 33.5inches. i should be building up a 58cm, but i didn't know this until i already bought frame. (someone told me once i am a 56cm. they were wrong) anyway. these 150mm and 200mm posts will definitely be too short. but am i safe with a 350mm? i saw one on the web i'd like to order. if no, then do you know a measurement in mm where i won't get to that safety line and still need height? p.s. i need to hyper extend close to 100% due to bad knees and my crank arms will be 175mm. thanks.

I'm still building up my new frame, but the Kalloy seat post I purchased from my LBS (whom I trust) is 350mm.

You guys need to worry about diameter as well...There are many different sizes.

Very true....but in my case, I took my frame to the LBS when I purchased the seatpost and had it measured (26.4mm).

my frame took a 27.2. after logging off, i just went to a bike shop and bought a run of the mill steel, alloy something or other seat post 400mm. the bike mechanic said the bike would have to be pretty small if that wouldn't get me up over my pedals. i could have had a carbon post for the same money off the web, but i'm in canada, so there would have been currency exchange, shipping and duty. thanks for your advice.