I drew a rectangle as in the Wiki manual and created the vertical and horizontal dimensions.
There are now smaller rectangles to the left and below the original rectangle with numbers showing the length in cm.
How do I get these new rectangles to look more like dimensioning lines, with the arrows on the ends?
This has to do with the zoom level / size of your objects.
Check out the "Dimensions" tab in "Current Drawing Preferences".
Adding this information to the manual seems like a priority update once the wiki move is complete.
I think the community would also benefit from a FAQ.'
An update could be to make the appropriate arrow and text size automatically calculated,
and allow for those options to be changed individually in the properties window.
An example set or table of recommendable / standard dimensioning settings for different purposes (mechanical, architectural, surveyor etc) and intended printing scales could be helpful for beginners.