Brake Stall: The rpm the engine cannot exceed with the brakes locked and the driveshaft not spinning. Brake stall isn't usually an accurate measuring tool since the engine often overpowers the wheels before the true stall speed is reached.
When a converter company quotes you a stall speed, verify whether it's flash stall or true stall. Many factors determine where the converter will flash stall once it's installed in your car. Heavy cars with tall (numerically low) gears and large-diameter tires offer more resistance to forward motion, so the converter will stall at a higher rpm than it would in a light car with steep gears and short tires. The easier the motor can accelerate the vehicle, the lower the converter will need to stall to get the car moving.
Of course, the power and torque curves of your motor will have a huge effect on stall speed. Generally speaking, engines that produce more low-end torque will bump the stall speed to a higher rpm. Conversely, the same converter will stall to a lower rpm behind a less torquey, higher-winding engine. Converter companies often designate the former as big-block and the latter small-block. When you buy a typical converter that's rated at 2,000-2,500-rpm stall, that rating is meant to span a variety of motors with different power curves. Scott Miller of TCI points out his company's 12-inch Saturday Night Special converters typically stall at 1,600-1,800 rpm behind a 325-375 lb-ft small-block and up to 2,000 rpm behind a 400-450 lb-ft big-block. Sure, they'll stall even higher behind a torquier motor, but they are intended for mild, conservatively cammed motors.
Miller explains that any converters size limits the amount of torque it can safely handle. Although larger-diameter converters have bigger parts, that's not always a good thing. Larger fins mean the fluid can exert more bending forces and result in failure. In stock form, bigger converters also allow more torque multiplication and lower potential stall speed, but the internals can handle only so much torque before parts start breaking. Reducing the diameter of the converter reduces its ability to multiply torque, puts less stress on the fins, and raises the stall speed, so small converters are generally better suited to peaky high-performance engines with higher-winding powerbands.
Other internal tweaks, such as fin angle and stator design, can have enormous effects on the converters fluid coupling, which changes stall speed and torque multiplication. So it's possible to build a tight 2,600-stall converter or a loose 4,000-stall converter in identical 10-inch housings, just by varying the internal design. Most internal mods are proprietary, but building a converter to achieve the right flash stall while maintaining around-town driveability takes a combination of proper stator design and fin angle in the correctly sized case. While it may be possible to build a 7,000-rpm 11-inch converter, it'd be horrendously inefficientstarting with an 8-inch converter housing would be a smarter choice.