Hi Stephanie,
You need to include the values "SizeForSearch" and "SizeRangeForSearch" in the selector to get the estimated number of users in a user list in the
google.com domain which are available for targeting in search campaigns. Could you retry the AdwordsUserListService.get() request by including these fields and let me know if you are able to get the size?
Thanks,
Shwetha, AdWords API Team.