The only "convention" as such that exists, is that lots of manufacturers happen to use Bach's sizing numbers (where a smaller number = larger diameter and the letter refers to the shape of the cup), if only because Bach is so ubiquitous in the brass world.
Here is a rather extensive chart:
Both Schilke and Denis Wick have mouthpiece comparison charts:
However, different manufacturers vary greatly in rim width/shape (not diameter, which is what is generally communicated in the 'size'), and that can have a much bigger effect on how the mouthpiece feels.
I find the best way to compare mouthpiece is to look up each individual manufacturer's product listing--Most have a comment of some sort that categorizes mouthpieces as being good for high range, open sound, free blowing, 'symphonic favourite', and the like.