How can I tell how many bytes a block of text in an email takes up?

Solution 1:

Drag the message from your email client to the desktop and open the resulting file (probably .EML) in a text editor that supports different encodings and shows the size of the data (eg Notepad2). Then you can simply cut the message before and after to see the actual size of the desired block (in the status bar for Notepad2).

Solution 2:

Notepad++ does this. Simply copy & paste the part of the message you want to size up into a new tab and read the length off the length field in the status bar at the bottom:

If you set the encoding to match your email client (Encoding menu -> UTF-8), it will count Unicode characters correctly (for example, ☃ is 3 bytes long in UTF-8 encoding).

This will only apply to plain-text emails, not to HTML or multipart/alternative emails.