Can Keyboard Maestro do simple select x# of lines & change the font size in a Mail email

Can Keyboard Maestro do simple select x# of lines & change the font size in a Mail email? How about select the rest of an email I need to forward and change it's font and font size? Thanks!

Yes.

Try sometihing like this untested, but common use case, workflow:

  1. Move cursor to start of current line (⌥↑)
  2. Repeat action X times:
    • ⌥⇧↓
  3. Use Select or Show a Menu Item action to change font name and size in Mail menu

Yes.
Replace Step 2 above with:

  • ⌘⇧↓

If for some reason you can't use the Select Menu to change the font, can:

  1. Copy
  2. Apply Style to Clipboard action
  3. Paste