Like some crazy-ass, no-lifer map making dude said (no offence ) : This is the reason why i didn't made that D&D map YET, but I still want to make a map right now,
So the idea is basically to make a melee map with heroes, just like war-craft 3. I am bad at melee map making and I wanted to rip off one of blizzard's map, so I came here to ask you all, what map would be nice for this kind of thing? (Preferably a map with at least 4 players that is smaller than 256*256, because I need to make it bigger to have some kind of area to trigger the heroes' abilities)

why don't you ask someone to make a map

More importantly, what are you going to do about the map boundaries? If you cut a normal blizz map that is say, 128x128 and your map is 196x196, then there are big chunks of the map that the players shouldnt be able to fly into.

Hyper Triggers and mass order stop blocks ?

But Vreal is right you should be able to trigger your hero's abilities without an outside area