Most of my 'pure' fracs are destined for online viewing only, so I don't do much more than 72 dpi at about 1200 pixels w and/or h maximum. But I do save the settings in case I ever need to re-render later for print.
If I am doing a frac for compositing with a Poser figure, I will go as big as my machine allows, so I can do post-render painting, etc. Then I resize downwards for the web.
It all depends on what I forsee as the pic's destiny. grin

RosemaryR
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"This...this is magnificent!"
"Oh, yeah. Ooooo. Aaaaah. That's how it starts.
Then, later, there's ...running. And....screaming."