I found Selwyn's script
'fullscreen resolution mode' thingy.
it's like auto fullscreen script, but it makes the game resolution 800x600 with fullscreen mode.
of course, if you press alt+enter, it comes back to original resolutino 640 x 480, but it is still useful.
the problem is, that script only changes the resolution but not the actual game size that comes out on the screen.
so the resolution is 800x600, but only 640x480 parts come out on the game );
and I found ForeverZer0's custom resolution script.
that one is so perfect.
you can change the resolution to any size you want, and it works!
but it's only in window mode.
It's okay if it is not possible making it work both window and fullscreen mode with changed resolution at once.
But, I really want the thing that works like ForeverZer0's custom reolution script, but works only in fullscreen.
in other words, "only fullscreen custom resolution script."
If anyone knows how, please help me.
I won't forget your help. thank you.