Zmud help

Is there a way to toggle how far I can scroll back?

There should be somewhere in your options a scroll back buffer you can change. Mine is 10000 lines I think. I don't use zmud but I remember this question from a long time ago when I needed to change mine.

Edit -> Preferences -> Memory -> Max Lines In Main Windows

There you go mate!

I tried that, it really screwed up the the format on the screen.

65K it the upper limit in the number of lines for zmud. Vobbe is right, and if your format got screwed, it could mean that you played with something else entirely.