Alot of people don't quite understand impeller sizing, here are a few pics that might help understand the concept.
Pictured are 2 impellers. One is a 'AAA" and the other is a "C". The impellers are the same with exception of the size. These 2 impellers would be drastically different as far as how they load your engine. You can see how they are very different on the large diameter here.
Changing impeller size only requires a few machining steps. You can trim down a larger impeller to a smaller impeller but you cannot go back, once its cut you are committed. A "C" cut impeller is very small and a "AAA" impeller is a very large impeller.
When you cut an impeller there are a few steps, first step is to machine down the large OD (Fig. A) This impeller pictured is a "AAA" impeller. To make this a "C" cut impeller we would turn down the large diameter as much as 3/4" making the large OD much smaller. By doing this Figure B gets larger and the water is released sooner from the impeller creating less load on the engine. In order to make this a "C" cut impeller you would also trim back the length of the impeller blades like you see in Figure C, again putting less load on the engine by releasing the water sooner. This is all done to the back side of the impeller, there are no changes to be made to the front side to change the rpm or load on the engine. If setup properly a "C" and a "AAA" could be interchangeable in your jet-drive.
Those are some of the basics to impeller sizing.