Thanks for the link to the information about the ISO standard. Interestingly, it doesn't actually specify dimensions, just number of characters. (Leastways, I didn't see any physical dimensions).
Anyway, I decided to create an A4 sized version by scaling the A3 version by the reciprocal of root 2 (= 0.707). Rounding the resulting dimensions to 0.5mm gives the result shown.
Somehow I managed to get into an environment where I could define a new title block. I managed to define the grid lines OK and started to create the small text labels. After doing two labels, Fusion just stopped and was totally unresponsive. I had to use Task Manager to cancel it.
After a restart I couldn't find the environment for creating a new title block, but I could find the place where I could modify an existing one. I started modifying the A3 version but, although I can redefine the grid and move the text labels, I can't see any way to change the sizes of the pre-fill targets.
All in all, it seems to me that Autodesk has completely ignored users who need to print 2D drawings on A4 sheets.
At the moment, my best work-a-round is to use an A3 sheet and output it to a PDF, then print the PDF on an A4 sheet. Unfortunately, the scaling is then root 2 times too big.
Where do I go from here?