What Is the Difference between a Soft Fork and a Hard Fork in Relation to Block Size Changes?

A soft fork is a backward-compatible protocol upgrade where non-upgraded nodes will still see new blocks as valid, though they may not understand the new rules. It can only enforce stricter rules, like SegWit.

A hard fork is a non-backward-compatible upgrade that requires all nodes to update to the new software. A hard fork is required to increase the block size limit, as old nodes would reject the larger blocks.