Sigurd,
I don't know of any way that you would limit the extent of the data in a service once it has been published. The limiting of features needs to be done at the time that you "Author Project for ArcGIS Server".
One thought I had:
If the majority of your data is reference info (which won't be edited) you could think about creating different projects of several extents (of background read-only data) that reside on the mobile device and never change (at least not frequently). Then, you could add the service you create which only contains the layers which will actually be edited. This way, they have different projects of related background data to limit the overall size but can add the same service of editable layers to each project.
Hope this helps.