Use the "fixed aspect ratio" on the selector tool, putting in the numbers for your final size. That will give you a box that while variable in size, will always have the same height / width ratio as your final thumb selection. Use that to select the area you want to make a thumb, and then crop, then resize to your final size.
Ta-da... you have a nice thumb.
I understand that some people do all their thumbs by hand... good luck to you.
Alex
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