i have some data on the size of forge island. i have tested using a ghost as seen here
going directly up
the ghost exploded at z: 395.8 which means z: 395.7 is the max safe point
going southward
the ghost exploded at y: -494.5 which means y: -494.4 is the max safe point
going northward
the ghost exploded at y: 487.5 which means y: 487.4 is the max safe point
going westward
the ghost exploded at x: 771.0 which means x: 770.9 is the max safe point
going eastward
the ghost exploded at x: -227.8 which means x: -227,7 is the max safe point
going downward, into the water
the ghost exploded at z: -70.2 which means -70.1 is the max safe point
to help you understand which directions are which, here are some pictures
West
East
North
South
Up
*directions were taken on the largest island*
Summary of the Max Safe Point Coordinates
X1: 770.9
X2: -227.7
Y1: 487.4
Y2: -494.4
Z1: 395.7
Z2: -70.2
max safe point is defined as: the area that a player can be in before Return To Battlefield is shown
happy forging!