Let say we want to have side by side 5 LEDs screens for a total of 1500x150 px, here is the method :
In "Resolution" put Custom -> the total number of LEDs (1500x150)
In "Tile Resolution" put the number of LEDs for one tile (300x150)
Now setup the pitch and margin parameters. The pitch will give you automatically the size of the led while the margin the distance between each tile.
Here is how to create this screen: