In the case of the Turnigy motors, 28 means the outside diameter of the can (which holds the magnets) and the 26 in this case means the length of the can. The 1000 is the kv and that is the number of RPM's per volt you should expect fromt he motor when it is run full throttle and not loaded with a prop. Using this number and the pack configuration, you should be able to size a motor properly. I typically run in the neighborhood of about 7000-10000 RPM with my motors, and I want to load the motor to about 70-80% of the no load speed with the prop (that's usually where the system is most efficient). So I target a no load speed around 9k-10k RPM. With a 3S battery (11.1) I use about a 900-1000 kv motor and with a 6S battery I ould use about a 450-500 kv motor. The 2826 part tells me roughly how large the motor is and how much current and power I can safely put into it.

I hope this helps, at least it is a starting point.
Curtis
P.S. When you are looking at Scorpion and AXI motors or the Towerpro motors, the 2826 would refer to the diameter and length of the stator within the motor (a 2826 AXI is considerably larger than the 2826 Turnigy)