You can use different grid sizes in subsequent calls to subplot without affecting the placement of plots created in previous calls. For instance, to create two plots across the top half of the figure, you can use:
subplot(2,2,1);
% plot something
subplot(2,2,2);
% plot something
Then to create 13 plots across the bottom half of the figure:
subplot(2,13,14);
% plot something
subplot(2,13,15);
% plot something
subplot(2,13,16);
% etc.
subplot(2,13,26);
