Let's put a sanity cap at a block line length of 255. Now, there's a brick limit of about 320K before your client can't keep up, and each brick can theoretically have up to 65535 events (I believe.) Now, let's put another sanity cap on event lengths at 255.
That makes the longest possible save size 255 * 65536 (one extra for the brick itself) * 320000.
That comes out to 5347737600000 bytes / 5222400000 kilobytes / 5100000 megabytes / 4980.5 gigabytes / 4.9 terabytes.
So yeah, Blockland saves can be freaking massive when you apply the worst case scenario.