Updated!
I fine tuned and found out the best value for 1x1 widget is 228x280.
It can resize to 2x2, 3x3 and 4x4 without problem.
But for asymmetry size eg. 2x1, 3x2, 4x2 they couldn't resize without gap.
Because gaps between array are not the same.
Automagic doesn't automatic stretch the widget.

For 1x1 widget, the best value is 228x280.
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For 2x1 widget, the best value is 496x280.
For 1x2 widget, the best value is 228x624.
For 4x1 widget, the best value is 1042x280.
For 4x2 widget, the best value is 480x280.