Hey there, @raycoms. =)

Yes, they can get large. Because they can get large, these worlds are limited in size and are super flat.

It is not possible to just throw any world into this addon and let it save to the Steem blockchain but rather having this addon itself create new worlds that are limited in size.

Small worlds are available to everyone who has a Steem Wallet (4 chunks size), bigger worlds need more resources and Steem Power to be broadcasted to Steem.

If somebody really pushes the limits, they might get a big world that contains 30 million blocks. But even then, the file gets compressed. The blocks don't get stored 1:1 into a file, but it rather searches for big chunks of blocks that are all the same type of block, then, it will be determined where this big chunk of blocks starts and where it ends. If we have for example 50k stone blocks at one place sitting around, it will be compressed to just one entry that says where the 50k stone blocks start and where they end. By not storing every block but rather using such techniques, bigger, custom created worlds can be stored on Steem.

While the size is still a limitation, it is enough to be creative to create challenging jump and runs maps or adventure maps. Saving the whole server, on which players play, including generated worlds, wouldn't work of course. But creating new worlds that contain just blocks which have been made by the players are possible. (superflat plot like worlds)