This topic has been touched on here and there, but my question is still looming. I have been using Ascension's tutorials to make my continent maps. The scale is 1 px = 1 mile. I want to know how I can "zoom in" or go to a closer scale such as 1 px = 1/8 mile (660 ft.). I have provided the particular map that I am currently trying to do this with.

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I have circled the area that I have in mind in the thick red circle. The entire image is 2800 px x 2800 px at 300 pixels/in.