at 8x12 you'll want to have at least 2400 Pixels by 3600 (which comes out to 300 DPI). You wouldn't go amiss doubling that.

In case you don't know how to set that: click on Render (next to Preview in the preview window) and then click on the dimensions. For me, they are both in the upper left corner of the window. Then click on Render to exact dimensions and just enter the pixels. You don't need to worry about the pixels per inch as it really has no effect on the finished project.

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