People tend to say that "XXX carb is too small for XXX" having no idea what they're talking about.
In an all out speed build, you can get more out of a bigger carb, but 15-19 is really the perfect range for the majority of bikes.
I think Jennings said for race applications you want a maximum of ~50% cylinder bore as your carb size. So for a gila'd E50, your ideal parameter maximum carb size is ~23.5mm. That said, for a daily rider you'll see a ton of benefits to running a 21mm carb or even 19mm, including better mileage, easier jetting, and increased low end response.