I'm going out on a limb and assuming i know which shop you were in. (initials IBC?) and i have to say that that shop sucks...they exists solely to sell you a bike...any bike (I also heard from an boston bike industry insider that they pay their mechanics and sales people crap and had some of their larger contracts with bike manufacturers terminated due to unethical business practices).
ANyways, here's my experience: i am 5"7' arms a little longer than normal. i ride a 16-17" mtn bike, no question about it, been fitted at ace wheelworks, bought a frame through them (16.5), pretty much knew how a bike was supposed to feel. Said shop tried to insist that my bike size was a large 18" intense tracer (very nice bike, not my size). The guy would not shut up about how great the bike fit me. i said it didnt fit me and didnt feel right, he told me that full suspension bikes are supposed to feel larger than hardtails (also told me hardtails were pretty much useless on the trails around boston). When i finally insisted that the size was wrong and asked if they had the next smaller size he told me that they didnt have a smaller size in stock. then tried to tell me that a suspension seatpost on my existing bike would "simulate" what the intense felt like (WTF?) I left after that...