How to prevent screen from resizing my Terminal in Mac OS X?
This will fix the problem: Bookmarks –> Manage Profiles –> Terminal Profiles –> Default
check "Disable session-initiated window resizing"
See also "iTerm resize the window itself when using screen".
Based on an Ubuntu forum post on problems with screen, I found that adding
termcapinfo xterm 'is=\E[r\E[m\E[2J\E[H\E[?7h\E[?1;4;6l'
fixes the problem, but I have yet to examine it to find out how.