Looks like you have set the Markdown editor as the default editor for your sticky notes.
For an existing sticky note, right click inside the sticky note and choose "Switch to Standard editor". Once you switch, you will be able to change "Font size", "Font color" etc.
You can set "Standard editor" as your default editor in Notezilla Preferences (Note tab). To access Notezilla 's Preferences, right click on Notezilla icon and choose Preferences from the menu.