Hmm in our code we used:
The default scale for physics is 2% = 0.02
So 400 / 0.02 = 20000
So yeah, 20,000 pixels is the max size.
I need to do a review of the physics code because the speed values dont conform with pixels per second like everything else does.
We originally wrote the physics using newton...and then we changed to chipmunk...and then we changed to box2d...so the codebase got a bit confusing trying to make it backwards compatable with older versions (so that force values all remained the same)
However so many of the games use physics, if I suddenly changed the unit measurement it would make lots of games behave wrong...I'll probably add a checkbox to use 'old unit scale' which will be ticked if your project is old