Scaling - good "standard" for score and parts?

Depends very much on the type of ensemble. For many situations, the default of 1.75mm for the staff space (which is a 7mm staff) is excellent, which is why they were chosen. Certainly for solo instrumental music or for most parts, it's a good size. MOLA guidelines for orchestra suggest going slightly larger for strings since they generally share stands.

For vocal music, you can afford to go smaller since the music is likely to be handheld as opposed to sitting on a music stand. For large instrumental ensemble scores, you want to find the largest size that still allows a full system to fit on the page. For smaller ensemble scores where you can possibly fit multiple systems per page, you want to find a tradeoff that works in terms of how many systems per page versus how many pages total versus how readable it is.