Was told the heavier the better. Why? Lighter lathes can do the job, but are less stable. One student grabbed the 10" lathe bed and was able to shake the lathe!
One gunsmith told us to get 15"! That's one heavy lathe with one heavy four jaw chuck. He says there more stable than the smaller lathes.
Mind you, the lathe Saint John Moses Browning had (that his father Johnathan hauled across the country) was a small lathe. Yet it got the job done.
BTW, I played with a small SouthBend that was made in Taiwan. Boo! Hiss! It's not like the old SouthBends that were made here back in the '40s. I'll take those old ones over a new one.