Yes fit can be a tricky thing, but it is important that the bike you are about to buy fits you properly. A bike that doesn't fit you makes for a miserable experience. For figuring out fit, you will need the following.

1)Tape Measure
2)A friend
3)A fit calculator, this one works: Fit Calculator
4)Which one of the 3 fits you are looking for that is in the link above.

After completing the steps in the fit calculator, it will provide the numbers that you need for a proper fit. Any of the makes will have the geometry table of the bikes they offer. Compare your numbers to what the numbers are on the bike's geometry chart and you will know what size you will need.