In warhammer 40000k how big does your custom map need to be and wats the max size?

Question

* note im referring to the board game.

Answer 1

The board game?  Custom map?  Nope, you've lost me.

Assuming that you're talking about the tabletop miniatures game, the size of the table you want to use depends on the size of the game.  For games of 2000 points or less (i.e. there are 2000 points of troops on the table; typically this will be divided 1000 each player) you will probably want to use a table no smaller than 4'x4' but perhaps as large as 6'x4'.

For games of 2000 to 6000 points, a 4'x8' table is probably the minimum size.  I suggest stopping at a 6'x8' table as, at that point, you have trouble reaching the middle.

WH40k:Apocalypse games can be played on tables up to 8'x12', with multiple players.  I've seen larger tables used, but these need to be arranged with a hole in the middle for one or more players to crawl into so as to move models near the centre of the table (assume the hole is impassable terrain that blocks all line of sight!).