1. To collect WebSphere metrics on the average size of an HTTP session, you must enable Average Size (b) Current in the Show columns list and then restart your WebSphere Application Server using the -Dquest.configfilter.httpsessionsize=true command.
Please follow guide - http://edocs.quest.com/foglight/5611/files/FoglightForJavaEE_Install.pdf at page - 28, "Understanding What Should Be Instrumented"
For more information about show/hide columns, see the Monitoring Application Servers User and Reference Guide - http://edocs.quest.com/foglight/5611/files/FoglightForApplicationServers_Guide.pdf at page - 11
2. This metrics is appears in the "Web Applications" dashboard under the "Session Settings" section in the table.
3. The parameter "-Dquest.configfilter.httpsessionsize=true" is a -D option. This should be set in the WAS startup script.
Edit the profile profile specific setupCmdLine.bat/sh script at WAS_HOME/profiles/<profile> bin folder and add PERFORMASURE_OPTIONS=-D
4. There is negligible impact - it’s just like another metric from the MBean server just like an additional jmx metric.