Then you would ALTER the column and SET STORAGE MAIN, so that it does not ever use TOAST.
The size limit for a row would then be 8kB minus page header minus row header, which should be somewhere in the vicinity of 8140 bytes.
If you want your block size to be a power of two, the limit would be 4kB, which would waste almost half your storage space.
Oh I see. So if I want to save some space for future columns, then storing about 7500 bytes in the "block bytea" column would be close to optimal, utilizing more than 90% of the block space. I guess that the fillfactor setting will have no effect on this table, and it does not matter if I set it or not.