Is it possible to change the font size of dimensions?
Yes, you can set a different font size for the dimensions in the Display Properties.
Select "Display Properties" on the "Options" menu, and then click "Edit", or use the context menu (right-click into the free workspace).
Then, you can set the "Font" in "General" -> "Dimensions" -> "Dimensions (see picture on the right).
In the "Screen" tab, you also find other dimension settings such as the line type, thickness and color.
If you have set several changes in this way, but you don't want to use them always, you can create and save different configurations in the Configuration Manager. There, those display properties are included in addition to other configurations.