There are a number of different theories out there to help determine the correct size lampshade for a lamp base. I think that often the best way is to trust your gut instinct, if it looks right then it probably is. That said, I tend to roughly abide by the following rules:

1. Measure the table/area where the lamp is to be situated. Will the shade overhang the table edge, is it likely to get knocked as people walk passed? Choose narrower or smaller lampshades and bases for console tables and thoroughfares.

2. Ideally the diameter of the lampshade should always be greater than the width of the widest part of the lamp base.

3. With that in mind, the width of the shade (A) should never be greater than the height of the lamp base (B) (from bottom to fitting). Ideally it should be roughly the same measurement.

4. The depth of a shade (C) should ideally be between half and two thirds the height of the lamp base (D) (from bottom to fitting).

5. The shape of the base often determines the best shape of the shade. A square/rectangular base looks better with a similar shaped shade. Likewise a rounded/oval base looks better with a tapered shade.

As I say there are no hard and fast rules, and after all rules are made to be broken. So go for it, get experimental!