Will stacking two magnets increase their magnetic field size?

Well, there'll be twice as much magnetic flux (hand-wavingly, twice as many magnetic field lines looping around, keeping in mind that magnetic field lines are a visualization technique, not something that physically exists). read more

Yes. If you let two equal bar magnets stick to each other by their attraction you increase the magnetic field outside them. This is known as the magnet’s form-factor effect. read more

But if you can push the magnets together like this -----→ 》NS《》SN《》NS《 ←----- Then you can increase the field and strength. but the field extends far in one direction only. Mythbusters had a show about using magnets as breaks. read more