Packs are typically in the 4MB-8MB range and are uploaded as the backup progresses. Periodically, and if restic is gracefully terminated (SIGINT, SIGTERM), it will write out a new index file for all of the content added so far.
Initiating a new backup after one has been interrupted should therefore deduplicate against the already-uploaded data. However, note that if this is the first snapshot, the backup client must re-hash all files locally. It will then skip uploading blobs that were previously uploaded.
Additionally, note that pruning the repository will discard all data that isn’t referenced by a snapshot, so pruning will effectively destroy the “resume data.”
(If your network connection is interrupted, restic will retry any failed operations for quite a while. If this is your only concern, you should be fine – restic is quite stubborn about trying to recover from failures.)