What effect does a larger or smaller diamter have?

I'd avoid anything cheaper than about $290, or made by TCI. I've had older TCIs that were great. But they changed after the business was sold. If this is a race car and you're interested in going fast repeatedly, the best convertor you can afford is the way to go. Contingency or not. As for size... I was told once that as the stall and lockup characteristics need to be at higher rpms, the diameter had to be reduced because of the way the oil had to react. Which was why in past years smaller meant looser. With better understanding of fluid dynamics and better control over the assembly process, a modern convertor can be slippry when neeeded and tight (within reason) when needed. Again, better convertor=$$$.