I'm using a plastic material and want to use the Roughness parameter to soften the reflections, but moving the slider to 0.001 (the minimum amount) softens them way too much. I've tried scaling the model up and down, and that doesn't seem to help. It's surely a units problem, but I don't see where that can be specified.
Any help would be appreciated, thanks.
Well, you should try a "good quality test render", and you will see 0,001 is almost without rougness.
Indeed, realtime render offers you only a sort of "preview" (in short time), but the final render will give you what you expect.
Try some tests... :)
Are you using a predefined plastic from the library?
It sounds like you may have the "sharp glossy reflections" option disabled.
First, go into edit>preferences and enabled "show advanced options". Now open the options window to the "environment" tab and you'll see a check box for "sharp glossy reflections", enable this and that should fix the issue.
Let us know if that's it.