17 minutes ago, spud42 said:why do you need diagonal fences? the bases form a square.
The outfield wall is an arc. That said, if we're talking about an impromptu sandlot field created by kids, they probably aren't overly concerned about the fence. I've got a sandlot field in my modlet on a 25x25 country residential plot, IIRC. I think the outfield "wall" is piles of trash, a washing machine, etc.
If you're going for an MLB field, then the infield is 27 blocks from base to base, 18 blocks from the pitchers mound to home plate, the outfield wall will be something like 90 to 120 blocks from home plate.