The default terminal size is supposed to be 80x25. Preferences->Current Profile->Appearance with "Size" option checked shows Width 80 and Height 25. I assume these are default values; there are no "width" or "height" in Profiles/Default file. The actual terminal size is 80x24 for new instances as well as with new windows from existing instance (Ctrl+Shift+N).
IF "width=80" and "height=25" are manually added to Profiles/Default, then terminal starts with correct 80x25 size.
Roxterm-gtk2 (not gtk3) 2.6.5 on Debian Wheezy (just pre-stable) with Openbox 3.5.0 (with and without Compton compositor).