How should I size my local buffer on a console server?

It's great that you are using your console server's port buffering/logging capabilities. It can be extremely helpful in troubleshooting a failed appliance. Ideally, you might want to consider sending the buffer files to an external source such as a SYSLOG server, especially if you are doing other types of monitoring via the SYSLOG. However, if local buffering is desired, the actual possible size of the buffer will depend on the console server you have.

Here are some guidelines:

Raritan SX – 64K
Open Gear – 4k in 4k out
ACS – limited to local flash size
ACS5000 – limited to local flash size
ACS6000 – limited to local flash size
Digi – 300k per port

Recommendations on the actual size per port will depend on: the target device you have, how much activity you expect from the console port, if the target device is okay after a certain point the buffer is replaced by new information, if you are buffering other ports, if you have an external SYSLOG. Unfortunately, there is not set answer.