Swot up on how eyeglass sizing works. Frames are given three separate measurements: the width of the lens (lens diameter), the width of the nose bridge and the length of the "arms" that hook over your ears -- they're actually called temples. So if glasses are sized 50 20 140, this means that the lens diameter is 50 mm, the nose bridge is 20 mm and the arm length is 140 mm.
Double the measurement given for the lens diameter of your coveted specs to account for the left and right lenses. Add the measurement for the nose bridge to this figure.
Take into account the length of the endpieces (the distance from the outside edge of the each lens to the outside edge of the frame) by multiplying the total obtained by adding together the diameter of each lens and the nose bridge by 1.1. Now you have a rough idea of the frame width.
Measure the total width of your head by holding a ruler up horizontally in front of your face. Note the measurement in millimeters.
Compare your glasses width and head width measurements. If the two measurements are similar, the glasses are likely to be an ideal fit.
If the frames have retro "winged" endpieces, you may need to multiply the figure you get in step 2 by 1.2 or 1.3 to get a better idea of the total width.
Trying on frames is the only fail-safe way to ensure a pair is the right size for you. For a perfect fit, you may need to take specs bought online to a local eyeglasses retailer for some minor adjustments.
If your existing glasses fit snugly, be guided by the measurements printed on the inside of the frame when shopping for a replacement pair.