The Lock Font Size toggle allows you to set the font size to be the same across all banners.
When the Lock Font Size toggle is On, all activated sizes will automatically change the font size in the associated box to match the text in the size you are working in. All sizes activated in the future will also adopt this font size.
Additionally, turning Lock Font Size on will make it so that the font size will be maintained when your text is added to another canvas as an embedded page.
For an example of how text will act with the Lock Font Size toggle on, please see the example below with an ISI Widget.
When the Lock Font Size toggle is Off, the font size of a text box is a Local function, so changing the font size in one size of your project will not affect other sizes. Additionally, if you set the toggle to Off, text in embedded pages will adjust when the embedded page is resized. For an example of this, see below.