I know that for LX Terminal this can be defined in the shortcut file by
specifying the number of columns and rows as parameters to the Exec
setting. That file is either in
/usr/share/raspi-ui-overrides/applications/lxterminal.desktop or
/usr/share/applications/lxterminal.desktop. Change this line:
Exec=lxterminal --geometry=120x60. But that's not what you asked ;)
Apparently for the file manager in LXDE the following works, but I haven't
tried it on a Pi:
Edit this file if it exists:
~/.config/pcmanfm/LXDE-pi/desktop-items-0.conf (it might be named slightly
differently? No idea about the conventions)
Or else edit or create this file:
~/.config/pcmanfm/LXDE/pcmanfm.conf (or maybe use that exact oath from
above?)
Edit or add these lines:
win_width=640
win_height=800
(They should under the [ui] category, or [*] probably works too)
For reference, the full configuration files are at:
/etc/xdg/pcmanfm/LXDE/pcmanfm.conf
/etc/xdg/pcmanfm/LXDE-pi/pcmanfm.conf
/etc/xdg/pcmanfm/default/pcmanfm.conf
(Maybe if the file in ~/.config doesn't work, try editing one of these,
with sudo.)