If you have two bar magnets, and one is larger, the larger magnet will have a larger magnetic field. However, if you are comparing different types of magnets (like the planet and hybrid magnet in Marcus's answer) the magnet's properties don't vary consistently with size.

A really strong type of magnets are "Electromagnets." Electromagnets are wire coiled around metal. When electricity runs through the wires, the electromagnet becomes magnetic. To separate objects that are stuck to the electromagnet, you simply turn the electricity off!

As you have heard the strength of a magnetic field tells you how strong a magnet is. When you play with magnets and you lift up things you use your hands to remove the objects from the magnet. But you are right. When you use a very strong magnet to lift up a car for example you could not use your hands to pull the car away from the magnet again. So, how do people lift up heavy things and then let it go again? To do this scientists have build magnets that can be turned on and off. These magnets are called electromagnets.

What electromagnets do is they place a ferromagnetic material such as iron inside a coil of wire with many loops. If you send a current through the wire it will produce a very strong magnet. If you turn off the current the magnet looses its power. So, if you have a crane with an electromagnet you turn on the switch and you have a magnet to lift up a car. When you turn off the switch the magnet lets the car fall off again.