Supernet block sizes - recommendations?

I recently received a request from one of our sites for a block of 256 network
numbers from one of our supernet blocks and have been hesitant to fill it
because of the relatively large fraction of the current NIC-allocated maximum
lock size (1024 networks) it represents. If I recall correctly, at the Boulder
Regional Techs meeting, we talked about "reasonable" block sizes in the 32 to
64 network range. Has anyone thought more about this issue or considered what
we should recommend either for the maximum block size to reallocate to our
clients or the maximum block size the NIC should allocate to us?