There is no definite resolution for games. Each game uses a different area of the Amiga's display and every game has a more or less big black frame around it. That's how it is on the real Amiga and that's how UAE emulates it.
The Amiga's display is either PAL or NTSC. PAL resolution is 720x568, so if your monitor can display it, set UAE's screen size to this, then what you get is as authentic as it can be.
Next best standard resolution is 800x600.
With WinUAE you could use DirectX filters to get the game's screen stretched to fit the display.
