18x20: This specification indicates two values. The first value is the diameter (ie. bore) of the cylinder in millimeters - it is usually 16mm or 19mm. In this case, the diameter is 19mm. The second value is the inter-axis (ie. distance) between the lever's pivot point and the plunger that pushes into the cylinder - it is usually 16mm, 18mm or 20mm. In this case, it's 16mm.
Now that we know what the numbers are, let's figure out what they mean in terms of braking performance. When you are selecting a master, you need to understand that these values trade-off braking sensitivity and braking power.
For the cylinder diameter, as that value increases, you increase your braking power. As you increase the diameter, you increase your cylinder size and increase the volume of brake fluid that you have to compress. This creates a dampening effect that allows you to better modulate the amount of brake pressure. As a general rule of thumb, you would use a 16xXX for a single caliper set-up anda 19xXX for a dual caliper set-up. Of course, there are always exceptions - for example, the stock master cylinder for Yamaha R1's and R6's (which are made by Brembo) use a 16xXX set-up, despite the fact that they have dual front calipers.
For the inter-axis value, as that value increases (ie. the distance gets longer), you are decreasing your sensitivity and increasing your brake power. I don't want to get into the technical aspect or into the physics of it...that's not the goal of this article. If you feel like you need to know more, I would recommend you search Google or How Stuff Works. In a general comparison between a 19x18 and 19x20 configuration (the most common configurations for sportbikes), a 19x18 has more feel but has a little more lever travel than the 19x20. A 19x20 configuration has more braking power and requires less distance to completely pull in the lever.
Ultimately, the optimal configuration is up to you. Brembo recommends the 19x18 configuration for racers and the 19x20 configuration for street riders. In terms of real world examples, the billet master cylinder using in MotoGP is a 19x18 while the master cylinder includes with the Brembo High Performance street kits is a 19x20.
Billet: Master cylinders are manufactured using two processes and are designated as either billet or forged. A billet master is cut from a solid block of aluminum using a CNC machine. This machine is computer-controlled and can carve out masters to very precise tolerances. A billet master is very precise but also very expensive to manufacture. A forged master created by heating a blob of aluminum (known as an ingot) to a temperature where it is malleable (but not a liquid) and pressing it into a mould to form the final shape. (It's different from casting where the metal is heated into a liquid state and poured into a mould.) The forging process is relatively crude and may have imperfections such as air bubbles or rough surfaces. A forged master is not as precise or refined as a billet master but is much cheaper to manufacture.
Non-Folding: Brembo masters are designed to absorb crash damage in one of two ways. The first method is through a folding lever. This lever has a hinge in it which allows the lever to swing up or down in the even that the motorcycle gets dumped. The second method is through a cut in the end of the (non-folding) lever. This is a deliberate weakness in the lever design - the lever breaks at the cut and absorbs energy. Sure, you end up with a broken lever but at least you still have part of a lever so you can use it to ride home. If you look at footpegs for rear sets, you may see the same idea at work. Breaking a lever is an easy replacement. Breaking the master cylinder means you have to replace the whole thing...not too good if you've invested in a Brembo.