Does the print-out use the full page or only a corner of the page?

If it uses only a corner of the page:

Thunderbird may have a problem accessing the correct paper size dimensions from your printer driver, or may have stored an incorrect size in the past.

To clear stored information about paper or other printer parameters, you can use the advanced configuration page.

app menu button (or Tools menu) > Options > Advanced panel > General mini-tab > "Config Editor" button

Click the button promising to be careful.

In the search box that appears above the list, type or paste print and pause while the list is filtered.

For your printer, there should be numerous bold lines with various parameters. You can right-click > Reset those to force Firefox to re-read your printer settings from Windows. (To speed this, you can right-click with the mouse in your right hand, and tap the R key to "click" Reset in the context menu.)

If it uses the full page:

Check Page Setup to make sure you have normal scaling (100%) set, and consider whether to turn off Shrink to Fit.

• app menu button > expand Print > Page Setup
• (tap Alt key) File menu > Page Setup

Sorry, on second reading, I think you meant print in the context of composing a message rather than printing it.

To check that your zoom level is set to the default, with your cursor in the message, press Ctrl+0 (that's a zero). This should make the font the size you are actually intending to send.

One way the font could have become tiny is if you happened to be holding down the Ctrl key when scrolling the mouse wheel toward you.

Regarding color, are you composing in HTML format? That should allow you to choose any color.

When you use plain text format, the recipient's settings control the display color.