I have a recipe that calls for a frying pan about 20cm across - should I measure from the base, or the top rim?
Manufacrurers measure the rim, so I guess the recipe calls for a pan which is 20 cm across the rim.
It may seem counterintuitive that the recipe specifies such a small pan (that's less than 8 inch for you Americans). But it can have good reasons.
Most recipes aren't dependent on pan size (much). But if you make a small quantity of some sauces, you want it to be thick enough in the pan to heat evenly, instead of a thin smearing of stuff on the bottom of the pan which will overheat quickly. So if you suspect that this is the case, it makes sense to actually use a small pan.
For many other recipes, overcrowding the pan is not too good. If you say what you are trying to make, we could give you an indication whether the size is important in your case.