When the key size is in the AJP request, it is passed to the javax.servlet.request.key_size request attribute as a String, whereas it should be an Integer according to the Servlet 2.4 spec.
I've set up a test using mod_proxy_ajp, the fragment of the request is 0x0b0100 (checked with Wireshark) corresponding to a key size attribute of value 256 (the cipher suite is DHE-RSA-AES256-SHA). The value of javax.servlet.request.key_size is "256" as a String, not as an Integer.
This seem to happen in org.mortbay.jetty.ajp.Ajp13Parser around line 409 in revision 3636. Unlike the client certificate information and the cipher suite, which have their own methods (parsedSslCert and parsedSslCipher) in Ajp13Parser.EventHandler, case Ajp13RequestHeaders.SSL_KEYSIZE_ATTR uses parsedRequestAttribute, which converts the buffer into a String.
This doesn't occur using mod_jk (at least the way I've set it up) since there is no packet with code 0x0b, although the key size can be deduced from the cipher suite.