Answer:
You can change the font size for dimensions under Display Properties.
You can access it using the menu "Options" → "Display Properties" → "Edit" or via the shortcut menu (right-click in the free workspace).
Under the "General" → "Dimensions" → "Dimensions" tab, you can then "Set Fonts" for the dimensions.
Further settings for dimensions, a line type, a line thickness, and color can be found in the same tab.
Note
If you have made several changes in this way, but do not want to use them all the time, you can create and save different configurations in the Configuration Manager. These configurations contain, among other things, the display properties.