Single player maps have no other limits that what a player's computer can run.
In multiplayer, however the limit exists because having many people download huge maps could severely slow down the servers. In multiplayer the limit is 8MB. With multiplayer it doesn't matter if it's LAN or B-Net.
I wonder if it would be possible to play if people already have the map...
It would definitely be very good if you could.
Wut?! of course if someone joins your game and he has the map he hasn´t to load it!
no... that not... you will get a message that says something like this:
this map is to friking big, to ensure waiting times of haters, we won´t let you create ir, please, go an play it alone you dumb.
you mean local games that uses platforms like gare or hamachi? surry guy they have the same limit than battle net.