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Great extension! How can I make the font size (of the main text) smaller?

You can create a custom stylesheet but of course you have to learn CSS syntax. Place it in the theme folder of your default style and reference it with Digests general settings in the ACP. Bear in mind that email clients don't always faithfully interpret CSS.

But it is in the digests.css?

You can call it anything you want, you just have to reference the stylesheet correctly in ACP Digests general settings.

Sorry, I mean in digest.css are the settings now?

As it says on the general setting page, it uses your default stylesheet, which is stylesheet.css. You might want your stylesheet to inherit from it (use absolute paths) and selectively override settings.

I am confusing you, sorry my fault.I want to change the font size of the mail that is sent.

Unfortunately, I can't teach you CSS. You probably need to change the CSS content class using the font-size CSS attribute, something like:
If you need help understanding CSS, please look at various CSS resources like w3schools.com.