Answers to your questions:
a) better put the cache directory to a dedicate disk, you can create the cache directory any where you want.
b) It really up to you, and adjust based on your site, maybe start from 500KB.
c) If you configure vhost natively, you can set that option at different level, for using with httpd.conf, it does not matter.
d) It is the same as "CacheMaxExpire", so just set appropriate value for your web app. how long you want one page being cached.
e) configuration in the default/main server section should be served as default value, you can customize it for each vhost.