If your current glasses fit well and are flattering, locate your frame size as explained below.  Select a frame with the same shape and a similar size. 

Frame Size Selection

The number in the frame listings is the eyesize or horizontal measurement across the lens of the frame.  The higher the number, the larger the frame.  If you currently have glasses, you can check the size on either the inside of either temple arm or the inside bridge of your current glasses.  It is a 2 digit number often followed by a square  and a second, smaller 2 digit number.  E.G.  50[]18 

Frame Shape Selection
A good frame selection can be simplified by considering what shapes are complementary to your facial shape. 

Oval Face - Most frame shapes are flattering, particularly geometric or rounded frames.   
Square Face - Rounded, ellipse or oval styles will soften your angular facial lines. 
Round Face - A full face, with very few angles.  Geometric frames will sharpen your features features and create definition. 
Triangle Face - A wide forehead narrowing torward the chin. Aviator or geometric styles add definition top the chin and cheek area.  
Heart Face -   A wide forehead, with high cheekbones, with narrow chin.  Rounded or geometric frames will minimize the width of the face in the temple area.