You cant change resolution in WinUAE on the fly, you have to set WinUAE's resolution before booting the emulated Amiga, save that into a configuration then boot the emulated environment.
If you intend the Amiga side to operate at 720x576 then the WinUAE resolution needs to be at least that to fit the Amiga screen in, say 800x600 centered.
With some PC side, driver tweaking you can get modern video cards to output PAL and NTSC resolutions and get a fairly faithful reproduction of a classic Amiga.
I think I'm answering your question?
