Anon, that's probably true in a lot of countries, but at least the slots can be all one size in this country. Many stores did not like the two dollar bills because cash registers weren't built with a slot for them--not because they were a different size.
Notes of the same size are easier to design for by cash register makers. Stamp machines and other machines that take paper money, I would think might be harder to design if notes were different sizes. When I was on RnR in Australia back in the '60s, I do not remember the different denominations there, having different sizes.
I don't see that many 50 dollar bills to know if what you say is true, I would think it would take some terrific slicer to split a bill. Also, some of the other stops to counterfeiting before our treasury department turned the cash into gaudy funny money, such as watermarks, color changing emblems, and a strip that identifies the denomination of the bill, are quick and easy ways to see what denomination the bill is. Clerks who are not paying attention, would be fooled by a copy machine copy of a bill anyway. Nothing fancy would need to be done to fool those types, unfortunately.
The more they make this money look like something from South Vietnam, the easier it is going to be able to counterfeit it. The more "busy" the engravings, the less people will look at the bills. They should just have a few things that are not able to being counterfeited, and stop messing with all this other stuff -- which the Iranians will be able to copy soon, anyway.