'wic help kickstart' documents --source and other partition types but doesn't mention empty partitions.
Moving to 1.9
Added this paragraph to the help text for --source option: If --source option is not used, the wic command will create empty partition. --size parameter must be used to specify size of empty partition. Here is the change: http://lists.openembedded.org/pipermail/openembedded-core/2015-June/106143.html
The fix is in master now: http://cgit.openembedded.org/openembedded-core/commit/?id=d92cda72ef605bf2f45437667c1fc36c76629d47
Verified on c1df471feacaf2590216aa476ce242908dac38cf.