when running lxterminal under fvwm, the first time you try to resize the window, it reverts back. Further attempts succeed. Using xscope I determined that the problem is that lxterminal waits for a ConfigureNotify before setting its geometry hints (i.e. terminal_vte_size_allocate_event) but it doesn't get a ConfigureNotify until after the first manual resize, and setting the hints resizes the window back.
I did a hack test where the hints were set instead of waiting for size-allocate and it seemed to work correctly (i.e. call terminal_set_geometry_hints instead of g_signal_connect("size-allocate"))