Sure, give it a try and see if it makes a difference. (My guess is it won’t.) Most people seem to stick in the 50-250MB range, though I’ve seen some go as high as 1GB for local storage.
You can change dblock size all you want and Duplicati will adjust (though only newly upload dblocks will use the new size, old ones will stay the size they were created with).
Assuming when you say “block size” you mean dblock (Upload volume) size then that’s a tougher question than it appears because it depends on a few other things such as your bandwidth speed and potential limits.
By default Duplicati will download 1 fileset for testing after each backup is completed. This means that if you choose a 75MB dblock (“Upload volume”) size then for each backup run ~75MB of your B2 download allocation will be used for testing files.
At 75MB that’s likely not going to be a problem, but I know some users who have used dblock sizes of 500MB then suddenly run out of bandwidth. So it all depends on how often you run backups, how many filesets you test per run (default is 1), and how much bandwidth allocation you have with your destination provider.
Also consider that when restoring files, the SMALLEST download chunk will be your dblock size. So even if you only want to restore a 1MB file, at least one 75MB block will need to be downloaded (possibly more if the blocks of your 1MB file are spread across multiple dblocks.
Oh - if you’re using a retention policy to thin your backups over time, you might also need to account for multiple dblock sized downloads during that process as well.
This page might help explain things a bit better than I did: