This is what I thought I would do.
I am going to assume the lowest res is going to be 600x350 and then allow the user to resize upto any maximum size. I guess you could always document that you decided what the resolution to be.
That said the resolution you have selected seems to be a bit extreme though, just to be on the safe side to avoid going through recorrections it might be a good idea to allow your GUI to handle lower resolutions. If the tester is running on a lower res it might cause a bit of hastle.
Thats my opinion anyways... hi res baah... miss the good ol low res 16color EGA days
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