When you delete any EBS snapshot, if there's a later snapshot for the same volume, every block in the first snapshot that wasn't included in the later snapshot (because it didn't change) is logically rolled forward (in a sense) into the later snapshot, so the later snapshot is still perfectly valid when any or all earlier snapshots are deleted.
Conceptually, you can also think of the snapshot process consisting of two separate pieces: the
compressed,¹ backed-up data that is stored in chunks in Amazon S3² during the snapshot process, and then the snapshot itself, which is only a container of pointers to those chunks of raw data. Every snapshot references a particular archived data chunk for each block of the volume, and those chunks are fetched and reassembled when a snapshot is restored. When a snapshot is deleted, any archived data chunk that is no longer referenced by any other snapshot is purged, but any chunk referenced by any other snapshot... isn't.
So you can freely delete any snapshot in a series, without impacting the validity of earlier or later snapshots. Note, though, that purging snapshots of volumes that don't change much will also not save you very much in monthly snapshot storage fees, because you already are paying very little for them, since they contain very little unique data.
How to determine actual size of an amazon snapshot?
¹ compressed ...well, maybe. The internal workings of EBS snapshots have always been a black box but when this answer was originally written in 2014, the conventional wisdom held that EBS compressed the backup chunks before storing them in S3. This may have been incorrect all along or may no longer be the case because it subsequently changed -- perhaps due to the fact that compressing already-encrypted data is much less efficient than compressing unencrypted data and EBS volumes are so easy to create with transparent encryption that the prevalence of encrypted volumes made the compression unproductive -- but there does not appear to be any official, documented source currently indicating that the backup data is actually compressed. The actual charge for snapshot storage is almost always substantially less than the logical size of the snapshot, so this particular detail is not necessarily important, but I've striken the word from the original answer, in the interest of accuracy.
² in Amazon S3 is where snapshot data is stored, but the buckets are owned and controlled by the EC2/EBS service, so the buckets are not visible in the console and the raw snapshot data isn't accessible to the end-user.