Just multiply the size you want the render to be (in inches) by 300, and set the resolution to that number. So, for example, if you wanted to make a 6" x 6" render at 300dpi, you would set the resolution to 1800 x 1800 pixels. That should work, right?
that's it, pretty much ... most people working with bitmaps only tend to think in 72dpi, unless they're working directly for print all the time
so, if you've got a test render at 800x600 (more or less A4 aspect, give or take a bit) that looks good, do 300/72 = 4.1666 and multiply the resolution by that. 3333 x 2500
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so, if you've got a test render at 800x600 (more or less A4 aspect, give or take a bit) that looks good, do 300/72 = 4.1666 and multiply the resolution by that. 3333 x 2500