Recomended because they're made by PMI as well.
Let's examine the 68/4500, the probably the most popular tank.
68 is the cubic inch volume of the tank. This one holds 68ci of air. A 45 series holds 45ci.
The second number, usually 3000,4500 or 5000 is the pressure. A higher number means it can hold more air in the same space. For example, a 68/4500 will last longer than a 68/3000.
For more information, look in the air systems section of the forum. There's plenty of info in there.