In reply to stuart58:
48cm is generally for rider height 5' - 5'4" but to answer your question;
Step 1
Take your inseam measurement to arrive at a general frame size. Stand against a wall with a book between your legs pressed firmly against your crotch, to simulate the pressure of a bike seat. Have a friend measure from the floor up to the top of the book using a tape measure.
Step 2
Convert the result from inches into centimeters by multiplying by 2.54 and rounding the result. An inseam of 29 inches, for example, converts to 74 cm.
Step 3
Multiply your inseam by 0.67 to get an estimated frame size, which is based on the length of the seat tube. For this example of an inseam of 74 cm, the approximate frame size would be 49 cm.
Hope this helps, Cheers, Mick.