56,57,58 mm. 60 mm might be getting more unusual. Not sure why a greater spacing allows for better barres unless your fingers just happen to like a touch more room. It will be a touch though. Increase the string band at the saddle and it has a very small effect on the string band at say fret 3. The right hand may notice it but between the thumb and the 'a' finger 3 mm over 60 mm is nothing, we aren't referring to the right hand stretch of a 13 course baroque lute! Of course there always comes a point when the straw breaks the camels back, I very much doubt that it's a 3 or 4 mm difference at the saddle unless you are at one of the extremes i.e. very fat fingers or very small fingers. It's at the extremes that very small changes can have a great effect on a player.