win98se screen size OOPS

My 8-yr-old son changed his win98se screen size (from 640×480) to something larger and apparently not compatible with his system/monitor. All we get now is fast/wavy lines and a completely illegible desktop.

Any ideas how to get it back?

I tried to stop it during boot, to go to SAFE mode, but I can’t get it… It’s F2 or F8 at boot, right?