Render to a file Question

I need to provide some pretty presntation drawings for mid october and i would like to "render to a file" i'm choosing Tiff file format.
however my question is when entering the "user defined" X,Y limits of the image, i'm having trouble selecting the prper numbers.
for those with experiance, what X,Y value should i enter to create an image that will plot @ 24"X36" with, say 300 DPI
i hope i am making myself clear? as i have never rendered to a file, i do not know the exact terminalogy.
anyhow, as the old saying goes, anyhelp is betterthen no help
Rock on!.. remember, its not work, its autocad!!
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