Comment 1 for bug 1117815

I am not seeing this:
$ dpkg -l|grep python-keystone
ii python-keystone 2012.1+stable~20120824-a16a0ab9-0ubuntu2.4 OpenStack identity service - Python library

$ grep max_param_size /usr/lib/python2.7/dist-packages/keystone/config.py
register_int('max_param_size', default=64)

In testing I also tried authenticating the with no changes to /etc/keystone/keystone.conf:
$ nova keypair-list
+-------+-------------------------------------------------+
| Name | Fingerprint |
+-------+-------------------------------------------------+
| mykey | ad:4f:2a:d2:c1:3f:8b:c0:e6:3f:de:cd:48:8d:fe:08 |
+-------+-------------------------------------------------+

and also with using in the [DEFAULTS] section of /etc/keystone/keystone.conf (and restarting keystone):
max_param_size = 4

$ nova keypair-list
ERROR: Request attribute username must be less than or equal to 4. The server could not comply with the request because the attribute size is invalid (too large). The client is assumed to be in error. (HTTP 400)

Can you provide the exact steps on how to reproduce along with your /etc/keystone/keystone.conf (redacting any passwords or otherwise sensitive information)?