"The big alligator is two meters long."
Translation:La granda aligatoro estas du metrojn longa.
This is called the accusative of measure:
Why wouldn't "longa" be in the accusative as well as "metrojn?" I guess what I'm asking is why can't it be, "La granda aligatoro estas du metrojn longajn."?
I'd say it was because it's in agreement with "granda aligatoro" rather than "du metrojn" like in "La aligatoro estas longa".
Two meters is not really a big alligator, is it? According to Wikipedia their average length is four meters
Because there's only one alligator. It's the alligator that's long not the meters.
because it's the singular alligator. The meters are not long. The alligator is.