You should prefer 1024 * 1024
Pros:
This image resolution can be downloaded to ldpi phones such as Samsung Galaxy Y.
2048 * 2048 will take 16 MB of graphics memory
while 1024 * 1024 will take 4 MB
so i think you can go with 1024 * 1024
Also, I would suggest that if you use images without an alpha value (Backgrounds), you can later hide .png to .jpg and edit the package file with the image name .png to .jpg.
Hope this helps u
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