Assuming you're just interested in filtering by the number of items in the contentarea (rather than the number of filtered items), there's a Count property on the ContentArea which should be indexed so you can filter using that property like this:
.Filter(x => x.CrossPublishing.Count.GreaterThan(1))
Normally I'd encourage the use of FilteredItems when dealing with a ContentArea however, in this instance we're dealing with data in the search index so, even if you index the value of FilteredItems.Count(), it won't return a meaningful value (i.e. one which takes visitor groups and permissions of the current user into account).
I have an article page type with a content area (CrossPublishing) that has only two other object types allowed to be created in it. Both of those two types have been decorated with the [IndexInContentAreas] attribute and in the Search and Navigation dashboard under the Overview I can see that the content of given content area gets indexed properly.
I want to filter on article pages with more than two items in that content area and with this piece of code
I always get 0 in Items and TotalMatching on the res object.
Is it even possible to use filtering on number of content area items or filtered items with use of .Count() and .GreaterThan(x) or .Match(x)?