Hi Ryan,
To be honest, I don't think this is possible. AFAIK, from admin console, it's only possible to set default pool size values for all MDBs globally. Per MDB, it's only possible to specify the values using the deployment descriptor.
However, you can provide an alternate deployment descriptor during deployment. This is supported in the "asadmin deploy" command with the "--altdd" argument. Therefore it's possible to tweak MDBs without repackaging, but it requires redeploying the app. The Admin console doesn't provide means to do that in the UI.
Ondro