Spaces

Pardon my ignorance but what are spaces equivalent to when it comes to measurement in Dorico? Spaces vs mm, inches, etc…

One space is the distance between the middle of one staff line, and the middle of an adjacent staff line. The actual distance in real terms is defined on the Page Setup page of Layout Options, where it is defined in points but can be shown to you in your preferred unit of measurement (which is set in Preferences). By default, Dorico uses rastral size 7, which is a space size of 4.96pt, which when multiplied by four gives a staff size of 19.84pt, or very close to 7mm or 0.28".