As far as I can work out a standard metric sandbag is 600mm x 300mm x 150mm
(L x W X H), while and imperial sandbag is 2' x 1' x 6", so knowing this and using an alternating brick lattice style stacking of the sandbags then this could give a result something like this:
I guestimated 750mm for the wall thickness in my pen drawing, but this could just as easily be 600mm, but there is also an obvious taper that would not occur with this style of stacking!
Looking at the smallest pen
(1) in the diagrams it looks like the internal dimensions are 48' x 37' , however I am unable to work out the wall thickness, other than it appears to be measured in inches and not feet and almost looks like 8" which is about 200mm !?!