The first thing to consider before replacing your glass lamp shade is what the proper "fitter" size of the shade holder is on the lamp or fixture. The shade holder is usually the part (most often made of metal) which will hold the glass lamp shade to the fixture.
The fitter is the portion of the shade holder through which the glass lamp shade passes through and makes physical contact with the lamp. Shade holders come in different fitter sizes and so do glass lamp shades. The idea is to match the same fitter size for both the shade holder and the shade itself.