As a Firefox support volunteer for the Thunderbird Support Forum, I can assist you with changing the font size in Thunderbird 102.12.0.

To change the font size in Thunderbird's calendar, you can follow these steps:

Open Thunderbird and go to the Calendar tab. In the calendar view, click on the "Settings" button (it looks like a gear icon) located at the top-right corner. In the settings menu, select "Options" from the dropdown. In the Options window, navigate to the "Display" tab. Under the "Fonts & Colors" section, you'll find the option to adjust the font size. You can use the dropdown menu to select a desired font size or manually input a specific value. After selecting the desired font size, click "OK" to save the changes. By following these steps, you should be able to change the font size in Thunderbird's calendar. If you have any further questions or need additional assistance, feel free to ask.

I must be blind, but when I pull up calendar there is no gear at right side as you suggested. The only settings I can get to it the general thunderbird setting and general account settings.

Do you have a font size issue in general throughout all of Thunderbird ? Eg: Folder Pane, Message List. Menus, Reading incoming mail etc etc OR Is the font size issue only in Calendar ? Where in the calendar?

The 'Setting' icon is shown in at the bottom of the 'Spaces Toolbar' shown on left side. The 'Settings' icon is also located under the Menu app icon which is top right.

However, changing the font size in the SEttings > General > Language & Appearance > Advanced area will only effect incoming mail seen in Message Pane and the font size in an opened Event or Task in the 'Description' area.

To increase general font size you will need to select the Folder Pane / Inbox tab first. If using top Menu Bar toolbar - View > Font Size > click on 'increase font size' If using menu app button - View > Font Size > click on + to increase the pixel size

Then select the Calendar and you should notice all the font size has increased.