I think (and I could be wrong), it is half the scale as Blender (by default), as when I make a default cube, it snaps up perfectly with Godot's grid on every other step (in other words, two default grid snaps in Godot moves the cube one Blender cube length).
Unfortunately, it appears Blender's default unit is known as a 'blender unit', at least based on my searching. I have no idea on a real life equivalent scale, but with some testing you may be able to figure it out. The easiest way is to make cubes at certain scales (like 1m, 0.5m, 0.1m, 10m, etc) and see if they line up nicely with the grid.
