Hello bramble,
We are not aware of any difficulties with changing the font size within the description editor, and I have tested the editor in my own account and was not able to reproduce an issue such as you have described, so it is unclear what may be occurring for you. Please try clearing the cache of your browser and then logging back in to your account and if the difficulty persists, please file a support case from under the Help tab of the site letting us know the details of what occurs for you when you try changing the font size in the editor and please include your browser and operating system information in the case so we can have a look into it.
- Craig