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Where am I going wrong here ?

SetWindowSize( 256, 342, 0 )
SetWindowAllowResize( 1 ) // allow the user to resize the window
SetVirtualResolution( 256, 342 ) // doesn't have to match the window
SetOrientationAllowed( 1, 1, 1, 1 ) // allow both portrait and landscape on mobile devices
SetSyncRate( 30, 0 ) // 30fps instead of 60 to save battery
SetScissor( 0,0,0,0 ) // use the maximum available screen space, no black borders
UseNewDefaultFonts( 1 ) // since version 2.0.22 we can use nicer default fonts
DrawBox(10, 10, 246, 332, 100, 100, 100, 100, 1)
GetImage(1, 0, 0, 256, 342)
CreateSprite(1, 1)
SetSpriteSize(1, 256, 342)
SetSpritePosition(1, 0, 0)
SaveImage(1,"box.png")

do
Print( ScreenFPS() )
Sync()

loop

I can save this box.png here like so - but the resulting image is 574 x 768, not 256 x 342 in size ?
(It's just a red box with a bit of a gap around the edges, almost full window size)

Just ran your code and it worked fine for me. I got the right size image but you were not using a proper "Color". Change the lines:

With this I get a box in the image at the size expected. As to why you get an incorrect image size I am not sure. Are you using a scaled windows setting?

Dunno if this applies but shouldnt there b a render() before getimage() ?

https://www.appgamekit.com/documentation/Reference/Image/GetImage.htm

Are you actually checking the correct file? AppGameKit is weird on where it saves stuff. On my computer, it saves to C:\Users\(username)\AppData\Local\AGKApps\(appname)\media.
If you are not sure, you can always look at the details of the image to see if it was created or modified at the correct time. For example, mine says Saturday, ‎October ‎12, ‎2019, ‏‎3:03:18 AM. If it instead said Tuesday, August 5,2004, then I'd know I was actually seeing an older image, and not the one that was just saved.