text size in sent emails

If you are using the Mail app, use plain text. Format > Make Plain Text. If that is how you're sending those email messages and they still appear to the receiver as having been formatted as something other than plain text, then something in between is reformatting the message.

Use plain or rich text in emails in Mail on Mac - Apple Support

Since it is not a macOS Mail problem, you ought to inquire on a Microsoft product support forum. Try https://answers.microsoft.com/en-us/outlook_com/forum