This is an old mod at http://www.phpbbhacks.com/download/1591 but you might be able to look at it and see if whatever it uses can be configured to do the job.  I entered in Google restrict phpBB signature size

Then I tried limit phpBB signature size images in Google and got this link:

http://www.phpbbsupport.co.uk/ftopic1333.php&sid=95dbfbcb4377a13bc75d7a8b381d81f7

Which had a link to this site's Signatures control 1.2.1 mod - http://www.golfexpert.net/phpbb/

The phpBB post about this mod found via the Search area there by entering signature images size limit and clicking the "Search for all terms" radio button - http://www.phpbb.com/phpBB/viewtopic.php?t=163493&highlight=signature+images+size+limit (most recent posts at end are this month, so this mod appears to have ongoing development and work with newer versions, but you'll want to read the whole thread as well as the mod's instructions to make sure it does in fact function right)

The reason I am providing how I found these items is to help users to determine keywords and search methods for future use.  Google is a great search tool, and you often find useful information even in the first few search returns.  You would then go to the forum in question ( in this case phpbb.com's forum at http://www.phpbb.com/phpBB/ ) to hunt further with applicable terms.  It took me less than 5 minutes to find all this information.

Glad you got it working for you