There is no standard. Some (probably even most) journals will have a specification of those sizes in their description of how to submit papers to the journal. For those that do not, you have to use your senses - when printed in the journal it has to be easily readable, yet not too large (definitely not larger than that of the font of the running text in a paper). I think you also should be able to feel what is right, once you have both text and plots together.
Luckily, using LaTeX, for example, this is a trivial matter to make right from the beginning. (Any standard LaTeX guide will tell you how to do it.)