Mail doesn't count characters but you can make a service that does it using automator.
start automator. it will give you a pulldown screen. choose to make a service on that screen. make it accept text and be available in Mail.
the workflow itself should consist of the following single "run applescript" action
<pre style="
font-family: Monaco, 'Courier New', Courier, monospace;
font-size: 10px;
margin: 0px;
padding: 5px;
border: 1px solid #000000;
width: 720px;
color: #000000;
background-color: #ADD8E6;
overflow: auto;"
title="this text can be pasted into the Script Editor">
on run {input, parameters}
set cn to count text items of (input as text)
display dialog "There are " & cn & " characters selected"
end run</pre>
save the service. call it something like "count-characters". then highlight the text you want to count in a message, control-click and select your service. it will display dialog counting the number of characters selected.
