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you could try a pencil and ruler and work it out for yourself and then compare it with the denis wick chart. I do remember these mouthpieces when I was young, they were not very good and we got rid of them fairly quickly, some got the Denis Wick (usually a no3 for an EEb tuba) or the well discussed Vincent Bach 24 AW, which was the route I went. I suspect that the Besson 13 was a bit smaller, more in line with a DW 4 or the VB 25. Alternatively, you could get your mouthpiece repaired and re-plated if you are that attached to it.

Steve,
firstly, I would say that the Besson New Standard EEb tuba is not a particularly small instrument, I used to play an Imperial, which apart from the valve caps was identical, if you have the 4 valve version you should have little problem playing down to the A 2 octaves below the stave, you may not need to for the music you play, but it does help with all the other aspects of playing.
To answer your question, a bigger mouthpiece (and I would state not the biggest possible and in the scheme of things, the DW3 is not overly large) will mean greater airflow and a better sound. Yes it makes playing high notes harder, but not impossible, and when you get them they sound so much better and really worth the effort. I would ditch the Besson mouthpiece and persevere wth DW3