Lxterminal does not appear to have any mechanism for customizing the default window geometry used when starting a new lxterminal process. It would be really useful if there was a system, analogous to the .Xresources functionality of Xterm, if a user's preferred geometry could be set up via the lxterminal config file, rather than having to give this on the command-line every time.
I've attached a patch that provides this functionality, based on the sources for lxterminal-0.1.11. (Note that the official sources seem to lack the file 'data/lxterminal-preferences.glade', against which this patch shows small changes to the preferences GUI.)
Patch to allow user customization of default window geometry.
Reverse-engineered .glade for preferences GUI