If a user wants to use:
#PBS -l size=192 ### Assuming you want to use 24 MPI tasks
aprun -n 24 -N 2 -S 1
Here's what the above aprun command means. You are asking for 24 MPI tasks, 2 MPI tasks per node, and 1 MPI task per socket.
At 1 task per socket, it is 2 tasks per node, so it will use 12 nodes (24/2) so the size would be 12*16 = 192. It is best to start with the aprun command to figure out how many nodes will be used, then multiply by 16 to get the value of size. Now, if you want to leave one socket empty for each node (use every other socket), then you would use aprun -n 24 -N 1, that tells it put one MPI process per node.