How to undo or delete Terminal command???

I have NEVER messed around with Terminal (and now I know why!)….

I was trying to find a way to finally have my mail message fonts show true to size, and found a supposed fix in this Apple Community here:

Inbox messages are very tiny. Can I zoom the type?

The fix was this:

1 - Quit Mail

2 - Open Terminal - in Utilities

3 - Paste the following into Terminal

defaults write com.apple.mail MinimumHTMLFontSize 14

4 - Hit return

5 - Open Mail

6 - Test

You can change the 14 to any size you want. You can adjust the normal Mail font size to a reasonable size knowing that you will always be able to read incoming messages.

However, what this did was now permanently make ALL my MacMail messages that I read and write default to Font size 14, and I cannot change them to anything else. I did not want to change my font size from 11, but to SEE it larger (or actually see it as font size 11 which it doesn't look that way on the screen but teensy)..

anyway, now I have NO IDEA how to undo or delete this Terminal command. I have no idea how to find this "bash" file or whatever it is called. I just do not want my emails all defaulting to one set 14 font and now disallowing me to change fonts as I write emails! For God's sake.

Please can someone tell me how to "undo" a command in Terminal and/or how to find this "bash" place and delete it or whatever….?

If anyone also knows how on earth to make your mail messages LOOK like the actual WYSIWYG you should see (as you do on all Mac things) that would also be awesome.

Thanks!!!!!