As far as measuring goes here is how I was told to do it:
How to Measure your Inseam: (Mi)
- Make sure you are standing up straight, on a hard surface, in bare or socked feet, with your feet 4 inches apart and your weight evenly distributed. For maximum accuracy and comfort you should wear your biking shorts.
- Measure vertically from the base of the pubic symphysis to the floor. Placing a book or similar size object in the crotch area, while exerting a small upward force, provides a good reference for this location.
- Make sure the tape measure is perpendicular (90 degree angle) to the floor, run the tape measure down from the upper edge of the book to the floor.
- Repeat this measurement 3 times, and then average the results for your inseam length.
- Round the calculated results to the closet whole number. (54.5 cm = 55cm)
Example: M#1 + M#2 + M#3 = Sum of measurements
Sum of Measurement / 3 = Average Inseam Length
Average Inseam Length * 0.67 = Road Frame Size
Taken from
http://www.plaines.com/page.htm?PG=fitguide