Thank-you. I assumed it would have a reasonable default. But it was at 0.1mm height for the text...and a similarly silly small number for the arrowheads. So although there, I couldn't see them.
The default text height for a new drawing with units mm is 2.5mm. If you open an existing drawing or use a template it might have different settings.
In the dimension settings there also is "General Scale". This is to easily adapt a drawing to a different printing scale without having to change all the dimension elements size individually. General Scale = 1 is for 1:1 print scale.