Version 2.7 of Tasks adds font size preferences. You can create additional font size options is you want.
Create a new CSS file with the settings you want to have, prefix the filename with 'font_' and put it into the 'css' directory. You can use the existing font size .css files as a starting place.
Every CSS file (filename starting with 'font_') in the 'css' directory is listed as an option in each user's Preferences.