Version 2018.00 adds an option to change the font size of the Details Pane. Since item descriptions can contain text of varying sizes, there isn't a single font size (in points) to specify. Instead, general sizes between "Tiny" and "Huge" can be selected. These sizes will scale the text larger or smaller. The default size is "Medium", as shown in the example below. There's no scaling when using the default size.
To change the font size, click the "Options -> Appearance" menu command. Next, click the "Details Pane" button and select the desired size from the "Font size" drop-down. The Details Pane will automatically refresh when a different size is selected, so you can preview the change without needing to exit the Options window.
Here's an example of how the Details Pane looks when "Huge" is selected: