Updated 2017-12-08
When you first create a new map in Hammer, you will have a blank black screen (possibly with a visible white grid). Usually, you will want to create a tilegrid first thing. Do this by clicking the "Tile Editor Tool" found on the left side of buttons on the screen.
Before the tilegrid is created, Hammer will ask you what size you would like to start with. Standard 64x64 is usually fine. The numbers are the number of tiles. Since tiles are 256x256 units (units being the standard measure for things in dota like the range of spells or the width of a model) and a standard map is 64 x 64 tiles (tiles being the standard measure for the placement of terrain) that meams we can calculate the number of units a standard map is by a simple arithmetic equation. 64 x 256 by 64 x 256. That comes to a Standard map = 16,384 x 16384 units. I believe that is the size of the default Dota 2 map.
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