Dekisugi

Set the maximum length of excerpt by number of characters or words.

Excerpt by Characters

Control the length of excerpt by the number of characters.

jojaba

This plugin replaces the word count info in bottom of the edit post window by the characters/words count (on the fly). Additionnaly, you can limit the characters/words count for defined user roles.
Here’s the list of the settings (see screenshots for further infos):
<ul>
<li>Enable or disable characters/words count limit.</li>
<li>Max characters and words count setting</li>
<li>Warning characters and words count setting (the count before max count when the warning is fired)</li>
<li>Output Format. You can define how you would like to see the output displayed using different placeholders : <code>#chars</code> (the character count that has been typed), <code>#words</code> (The number of words), <code>#maxChars</code> (the max characters allowed), <code>#leftChars</code> (the characters count left),<code>#maxWords</code> (the max words allowed), <code>#leftWords</code> (the words count left).</li>
<li>Choose what user role should be limited. Default set to contributor role.</li>
<li>Choose the post types that should be limited. Default set to post.</li>
<li>Set customised messages for warning or for contributor submission.</li>
</ul>
Availabe languages : english and french.

WordPress Word Count and Limit

Counts dynamically the characters/words in edit post window and limit the characters/words count if needed for one or more user roles.

Excerpt by Characters vs WordPress Word Count and Limit