render to disc at whatever size required at 300 dpi to get the best quality.
rule of thumb..

1. if you lower the dpi..you can increase your image size..oh photoshop, that is.
2. you can increase dpi if only you reduce the image in size.
3. you can Never ever increase size and not reduce dpi.

put it this way..stoner..imagine pixels as being fluid in a little confined space.

you squash the space.. ie..make the image smaller.. keep the same amount of fluid..ie..dpi..and well.. the color stays good.
imagine widening up that space..asking the fluid to spread itself out.. of course..it's gonna look really spread out, very little honey over a slice of bread. can't cover the whole area etc..ie..pixelated.

and the rest..you can figure it out...

have fun...