Originally, there was no limit.
I thought it was 32MB (and was later reduced to 1MB) - are you sure no limit?
There was a 32 MB limit on maximum network message size. This applied to anything sent in a single network message. Since, at the time, a block was sent as a single network message, there was indirectly a 32 MB limit on the size of the block, but there wasn't any specific size limit specifically intended for blocks.
Then the MAX_BLOCK_SIZE limitation was added to the code in July 2010 with a 1 MB limit.