Ah :)
I just work on a pure pixel basis since it's a direct conversion and you don't need ot worry about such things.

Divide your pixel size by required DPi to get inch sizes.
3000x3000 pixels = 10"x10" at 300 DPi.

A pixel literally = 1 "Dot" from DPi. It's the pixel size which is most important in digital images, because that is the true absolute resolution of it. Physicla size (inches etc) isn't important, only resolution is.
Where as DPi or equivalent is the resolution of film, or a canvas or whatever.

For big renders, you are as well doing a large-ish render, rather than an enormous one, minimum of 1600 or so IMHO, and use Photoshop or one of the good photoshop plugins for scaling up, "Genuine Fractals" or the like,

I have the trial version of Genuine Fractals on my art PC to test it, and it was actually good, I could see the way it was an improvement on scaling up that Photoshop can do. Bit pricey though so I'll need ot wait before buying that
http://www.ononesoftware.com/detail.php?prodLine_id=2.

I used Photoshop itself  to scale up some of my pics for printing out at 30" x 20" and they looked good :)

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