How do I enter a specific size with the dimension tool? I'm trying to enter 30mm

As I said before, unfortunately I don't know your settings.

But maybe it helps to deactivate "Use Preview Bounds" and draw your rectangle exactly 30 mm wide?

@packaging specialist 

I would not do this (unless it is absolutely necessary for some reason). Because after that the measure is no longer live.

Hi @James3669094303ct 

Why do you need a different measurement than your object has? What settings are you using?

Expand apperance then you can enter any value