I got a very nice Begonia Kellermani from you. After a year it is huge but "leggi". I'm growing it as an inside plant. Can I trim it. Some sites say trim it back to 4 inches from base?

We generally suggest not cutting off more than 1/3 of the total plants growth as it can stress the plant out. Start small :)

You should be able to trim it, however, care needs to be taken to use a sharp knife, and disinfect after each cut. Also, don't trim too much at a time, as this will stress the plant. Begonias require filtered sunlight, as they are an understory plant. Could be why it's leggy, too much sun.