GIMP Tutorial for Beginners

I am trying to resize a picture for a mug template on gearbubble.com.  The dimensions of the picture should be 3 x 3.5 inches which converts, I thought, to 900 x 600 pixels.  It appears to still be to big for their template.  What did I do wrong?  i do have a separate template to open in gimp but unsure how to use it in layers.  Can you provide some steps on how to do this?

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