Measure the inseam length of your legs (barefoot). You need a large format hard cover book. Stand straight up, with your back against the wall. Place the book between your legs as shown in the illustration. Make sure the the book is rectangular against the wall. Use your hands to pull upward on the book to simulate saddle pressure. When the appropriate pressure is reached, hold the book in position. Make sure the book is square with the wall! This may require some help from your assistant. When the book is in the proper position, your assistant should measure from the floor to the top of the book as illustrated, and write it down. Perform the entire procedure at least two more times until you get a fairly consistent measurement. Average your three consistent measurements.
The inseam lenght decides if you can use 28" or 26" wheels or have to switch to even smaller front wheels like 20".
The table on the Wheel Sizes page gives a good overview of the possible combinations.