If you take a typical lathe spec'ed, say 36" between centers that will be the max 'between centers only', and most of us are using chucks more that centers. Put on a chuck and plan on losing 6" so a 36" becomes that 30".
Larry, I admire the work you do on that small Atlas! (I started with one and have had 3) As you well know and a wise man once said," you can do small work on a big lathe but not always so the other way around". My unit for the past 30yrs has been a Craftsman 12x36 w/o the gearbox. No GB for small threading (unless trying to duplicate proprietary threads) is more quickly done with taps and dies supported in the appropriate fixtures. So, for a lot of gunwork a GB isnt nessesary BUT, a large dia. spindle hole is. Mine is too small, as are most of the older, and affordable benchtop lathe units. This means that one must chuck up a barrel 'overbed' rather than having some of it through the spindle. What Im getting at here is that if you can run larger dia. stock through the spindle you can get by with a shorter bed for a lot of work.
For anyone who really wants to learn this stuff I suggest taking a course at your local Voc-Tech school, say Machining 101, etc. You will gain invaluable experience, MAKE some stuff, and learn what you need for determining what size machine you want to get into.
There are some nice used machines out there (and a bunch of junk). Its good to know how to tell the difference.