If the converter measures 15 inches in diameter, the impeller would need to spin only to a low, say, 1,000 rpm before the car would begin to move slowly. However, a 7 1/2-inch converter in the same car might need to spin to 5,000 rpm before the car moves, because that little unit needs to overcome the resistance before it will accelerate the car. Artificial resistance can be created through a trans brake, making the impeller spin higher against the locked driveline. In addition to increasing or decreasing the physical size of the converter, the builder even more closely regulates the desired stall speed by adjusting the stator's sprag spring strength and vane angles in the impeller, stator, and turbine.
A stock converter's vanes are normally static, or vertical, when assembled. The builder can create a negative angle to increase stall and resistance by bending vanes forward so the fluid has to work harder to get the turbine to spin. Therefore, say a 10-inch converter that stalls stock at 2,400 rpm can be made to stall at 3,400 rpm. The blade angle can be bent in the opposite direction so the converter becomes more efficient.
The trade-offs are driveability and heat. The faster the fluid is spun in the converter, the hotter it gets. Even though it's transferred through the front pump of the transmission and interchanged with the fluid in the transmission so it's "fresh," this is why an aftermarket transmission cooler is very important on performance cars with high-stalling converters. Moreover, a high-quality fluid is mandatory to ensure it doesn't burn due to the increased heat (some synthetics are now good to temperatures above 400 degrees Fahrenheit, and will give additional stall to boot). Heat is the most common factor for transmission failures, as the fluid breaks down when it's overheated and loses its hydraulic optimum.
Driveability suffers as the converter slips when there isn't enough rpm and fluid force to turn the turbine effectively; the impeller is spinning, but not hard enough to allow the turbine to spin at the same rate. Therefore, the fluid is taking the brunt of the horsepower and creating, you guessed it, heat. If the converter stalls at 3,200 rpm and you're driving at 2,200, the car may be moving, but a lot of energy is going right to the liquid in the form of heat and the stator remains locked. All converters will slip to some extent. The higher the stall, the more slippage, so there's often a trade-off in mph with a given combination when going to a higher-stalling unit.