I hope you got it figured out in time.
I know it's a bit late in the game but 8x10 and 11x14 are different aspect ratios.
That means the relationship of the long side to the short side of the sizes is different.
The long side of an 8x10 is 1.25 times the short side.
The long side of an 11x14 is 1.273 times the short side.
Mpix's minimum resolution is 100 ppi so the 11x14 has to be at least 1100x1400 pixels.
Keith
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