For MTDv1, there are usually two main types of f3s_sync() callouts: one for boot-block flash and one for regular flash. The boot-block flash have two different sizes for erase blocks. The boot-block f3s_sync() callout needs to know the size of the block it wants to erase. The regular f3s_sync() callout doesn't need this extra logic. If you are not sure, pick the f3s_sync() for boot-block flash — it is slightly slower, but works for any kind of flash.
If you need to use a f3s_v2sync() callout, there's usually only one to choose from.
The f3s_v2sync() callout doesn't need to know the erase block size. The size is determined by the filesystem via a different API.