Please see Choosing sizes in Duplicati. Beware of naming – above comment fits –dblock-size not –blocksize.
Regarding download charges due to the sampled verification, it’s adjustable all the way down to 0 if you like, however you’d increase risks of a bad backup unless you took steps periodically to test / verify your backup.
Understanding B2 Pricing Structure makes me think doing verification in small chunks might wind up at free, and the Duplicati job report will tell you how many bytes it downloaded (can’t say other apps’ use of course). Note that downloads will also happen when the automatic but adjustable compact operation reclaims space.
Forum comments are kind of mixed on whether the –backup-test-samples counts source or destination files, however from source and tests, it looks like a sample is a triplet of a dlist, dindex, and dblock file. A dblock is capped in size at --dblock-size, the dindex indexes the dblock and is usually small, and the dlist contains the pathnames in the backup (plus information to allow restore). For a big backup, that file might be the biggest.