The length of my drone is 14 inches…what size blades should I use and where can I acquire them?
That question can not be answered.
A multicopter is a very complicated device. The relationship between the motors, the props and the weight are intertwined and can not be looked at without all the system at once.
The general method is to first identify the load. How heavy is your payload, then how big a frame to you need to support that load. Then what motor/prop combination can produce the thrust needed to overcome the load.
One tool, among many, is eCalc (www.ecalc.ch) which will take all the variables and make predictions. Then you may have to modify one or more parameters and recalculate iteratively until you find the best solution.