Why not think about it? Look at the inside.
What does this:
do? If you have no clue, then READ THE HELP for size. Suppose you broke it into two pieces:
K = size(session,1);
something = cell(K,1);
This is essentially identical to the fragment that you show, in terms of what it will do, except that it also creates a spurious variable called K. But that is not really relevant. If you know what size does, AND you understand what the cell function does, then what does it tell you?
When you see a problem too difficult for you to understand, break it down into pieces.