"The large coffee is bad."
Translation:La granda kafo estas malbona.
"larĝa" means "broad" and is not used the same as "granda" which means "big, "great" or "large". (Large in French does not mean the same as large in English either.) http://esperanto-panorama.net/vortaro/eoen.htm
This dictionary lists "huge" as "grandeja" (EDIT: No it doesn't, my typo - I should have trusted my instinct that it was wrong. and looked it up again. See comments below.), but "-ej-" is specifically relating to places I thought. http://www.gutenberg.org/files/16967/16967-h/16967-h.htm
Kafego estus tre kielkafa kafo, ne granda kafejo.
"Kafego" would be very coffee-like coffee, not a large café. And "kafejego" would be a very café-like café, not a large café.
-eg- intensifies the word it's attached to, it is only to do with size in size words "grandega" "larĝega" etc. Think of it like the word very.
Grandego = very big (huge)
Bonega = very good (great, excellent)
"Estas" links two ideas together, just as it does in English, as a present tense verb (a "copula"). "La granda kafo estas malbona" is a statement about the state of the large coffee. There's no implied action here, just a description. That's clear enough, I think.
In "La granda kafo malbonas", what's really happening here is that you are not adding an -s but adding an "-as", that is, changing an adjective (malbona) into a present tense verb (malbonas). The semantic sense is the same, but the flavor of the sentence, if you will, changes greatly. The large coffee is now actively being bad rather than just in a state of being bad.
An attribute of Esperanto is that any word root (including suffixes and prefixes!) can, conceivably, become a verb as well, if doing so is sensible within a sentence. This tends to make Esperanto sentences more active than we might have them in English where we want to separate subject and action. To say in Esperanto, "La ĉielo bluas" has the same idea as the English sentence "The sky is blue" but with added gusto: the sky is doing something here -- bluing, for the lack of a better concept in English. One could say, "La ĉielo estas blua" and why not? But "La ĉielo bluas" gives agency to the sky, something we don't have in English.
In this case, the coffee is more actively bad than it can be in English. So maybe it's not Starbucks?