Hi Gysmo
Not sure if I understand your question correct, but creating var-regions via remote admin of jOpenSim is currently not supported.
When creating regions via jOpenSim, you will find them in your OpenSim as ./bin/Regions/jOpenSim.ini
to change size of a region simply add e.g.
SizeX = 1024
SizeY = 1024
to this region ion that ini file and restart the simulator.