From page 575 of the Nuke 6.0v6 User Guide:
http://www.thefoundry.co.uk/dl_file.aspx?ui=3A36E7BB-45BD-48AA-88EE-52CD6D03221E
"""
NUKE_DISK_CACHE_GB:
The maximum size the flipbook cache can reach (in giga-bytes). If this variable is not set, the location is determined by the flipbook cache size setting in the Preferences.
"""
So, presuming you're using Linux that would mean adding this environment variable to your login profile and/or to a script you use to launch Nuke with pre-set options. using bash it would be:
export NUKE_DISK_CACHE_GB=20
As for 'memory usage' you might want to look at NUKE_DEBUG_MEMORY in the same documentation.
/p