Edit: Corrected my answer. The mentioned winsize sessionoption only refers to the vim internal window layout, not the external dimensions.
If you use
:mksession
and load the session on vim startup with
gvim -S Session.vim
you can include the window position and size into the session by including winpos and resize in the sessionoptions, see
:help 'sessionoptions
With autocommands you could even automate the saving and restoring of the session on Vim entry and exit.