This question already has an answer here:
I am presently riding a hybrid that apparently is a bit too small for me, and I'd like to replace it with a road bike. Having ridden this improperly-sized bike for as long as I have (about 8 months, including one MS150), I am interested in making sure that the next bike I buy is properly sized.
The sizing numbers published for different bike models are only somewhat helpful, because a 54cm bike, even of the same type, will fit different people depending on who made the bike. It would have been more helpful if the sizing was listed as a range of heights for riders (such as, "best fits riders from 175 to 185 cm").
Somewhat complicating the issue is that (as I have mentioned in another question) I am chronically short of funds, and there is a high chance that I'll be buying something off of Craig's List or some other such place. I don't want to waste the seller's time by trying the bike if there's any chance that I won't buy it. I'd much prefer to know when I show up to make the purchase that the bike is manufactured for a rider of my height.
The same goes for mail-order. If I go this route, I want a reasonable certainty that the bike is made for someone my size. I don't want to deal with the hassle of returning a bike that doesn't hit my own personal Goldilocks zone.
I'd also rather not trouble bike shop personnel to let me try a bike to see if the sizing is right unless I'm ready to buy from that shop (possible but by all means not certain); I like to think that I'm a golden rule sort of person and using their time in this way seems too much like taking advantage of them. However, if this is considered by them to be a cost of doing business, then it's not a show-stopper.
That all being said, what's the best way to go about finding what size bike will fit me best? (If it helps to be more specific, I'm mostly interested in a road bike.)