those "blocks" are just the amount of data mIRC sends before it waits for the other client to tell it the data has been recieved before sending more data.
These "blocks" are called packets, and every program which uses tcp/ip uses them to send/recieve data. Thats the way the tcp/ip protocol works.
The whole /dcc command and DCC sending would have to be rescripted on both clients to allow for curruption checking,
