That sounds weird, 300×100 is a really strange setting. Some DOS games did really strange stuff with mouse implementation though, so in some cases, it’s best to just use relative mouse.

Sadly, I don’t own the game myself to experiment with. BUt I know that in some cases, the mouse is only messed up in menus, but working ingame. If that’s the case here, too, you could e.g. use relative mouse to navigate the menu, hten switch to absolute ingame. There’s also a Deadly Strike widget to automate this switch.