Step1 Measure your inseam. This is best done barefoot. Stand against the wall and put a book between you legs so it pressed right up against your pelvic bone. Make a mark with a pencil on the wall along the top of the book. Measure the distance from the floor to the mark in centimeters.
Use your inseam measurement to get a rough idea of your road bike size. Multiply your inseam by .65. This will give you a good estimate of you road bike size for bikes measured center to center. Thus if your inseam is 86 cm, you will fit a 56cm road bike (86 x .65 = 55.9). Note that many road bikes are measured center to top. To determine how to fit these bikes, multiply your inseam by .67.
Subtract an additional 10cm and convert this measurement to inches to get your mountain bike frame size, roughly. Thus, if you fit a 56cm road bike (c-c), you will fit a 46cm - or an 18" - mountain bike.
Consider top tube length. In many ways, this is the most important aspect of sizing a bike. Two 18" mountain bikes may have different length top tubes. Or a 54cm and a 56cm road bike may have the same length top tube. Given the same top tube length, the bigger bike may be more comfortable in that it will allow you to get the bars up a bit higher.
Know that women have longer legs and shorter top tubes than men. Unfortunately, most bikes are designed for men, and women often have a hard time finding a bike that will fit them well. Luckily, some manufacturers have begun to make women-specific models.
Test ride some bikes once you have gotten a rough idea of what will fit you. This will help you determine what is most comfortable for you.