How to change the side bar folder and dashboard font size

This is a workaround solution, not an actual configurable feature via the plugin

Log in as an administrator into Jira
Select User Interface > Announcement banner in the System panel or use the shortcut 'g'+'g' and type in "Announcement Banner"

Enter the relevant CSS below into the Announcement field:

<style>
#dashboard>.tabs.vertical a, #dashboard>.tabs.vertical li.active {
    font-size: 15.5px;
}
</style>

<style>
#d_folder_nav > li > a.ddFolder.dashboard, #d_folder_nav > li > ul > li > a.ddFolder.dashboard {
    font-size: 14px;
}
</style>