• Specify a dropdown with KB, MB, GB to be more human-friendly.
• If using just B / bytes, we should round the size up to 4096 B, or a multiple of 256 B as documented:

https://docs.mongodb.com/manual/core/capped-collections/#create-a-capped-collection

Note: An earlier version of this ticket confused the Oplog with capped collections in general, and has been updated accordingly.