I really doubt that you can. It will be dominated by the sizes of the partitions (there may be a few system files in other places, but it will treat the entire partitions as "used" space, which is fair as you can't access that space). Resizing the partitions would be a specialised and risky modification, going much deeper than just rooting the phone, and with a fair chance of breaking the device, either immediately or when it tries to install an update. Maybe it would be possible for an advanced modder to remove one set, provided you never wanted to update the software again (not even a security patch), but I doubt that anyone has even tried.
You would need root even to see how much free space there is in those partitions (which would be the maximum you could possibly gain by resizing). All I can say is that my Pixel 2, which also has A/B partitions, is using 9.4GB for system storage, which isn't much smaller.
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