Over 700 atoms is certainly pushing the limit. There is no strict limit, since it depends on your parameters. The memory consumption depends critically on the k-point sampling, the basis set size (and of course the number of atoms), and the grid size (total simulation volume, taking the mesh cut-off into account).
Also the parallelization strategy is important. I see you use -n 8, but do you have 8 computers, or one machine with two quad-cores? Each MPI processes uses the same amount of memory, so if you run 8 processes on one single machine, each process can max use 1/8 of the machine RAM. In this case, you can only run small systems - or use fewer MPI processes.
