Te first argument is usually a variable already declared one more more lines above your cursor, so you can start with that. Then your IDE kicks in and gives you a list of possible methods. In this case it is easy, if you want to use "length" or "size" for your list.
The "dot-autocompletion" has a pretty good usability in my opinion. You write down the name of the variable, enter a dot and you get a filtered list of methods that "make sense" to call on the receiver (key word: discoverability of the API).
How exactly would one implement the trigger with multimethods? I think this is an important consideration to make from a usability standpoint.
I admit it's a little weird, but not so weird people couldn't get used to it.
x y x DoSomething