Snapshots are block level. So... technically yeah, if 100% of the blocks are changed between snapshots. you'll need to account for 100% of the blocks.
No space is "required to take the snapshot"
Though, I would do the 7GB thin volume to give yourself some breathing room. so 1.5G for the file space and 5.5 for the snapshot space. The snapshot % is adjustable anytime, so you can fiddle with it after the fact if you want too.
when you take a snapshot, no space will be taken immediately. when changes come in to the original data blocks, the original blocks will be moved to the snapshot area. if your data is 1GB, the snapshot will hold all changed blocks until next snapshot.
volume can be expanded anytime as you need, you can give 2GB to start, if not enough, expand the size. if thin provisioning is being used, you will not waste any space.