Can you say "guy" for chłopak?
We do accept it in some sentences, I guess it suits this one. OK, added.
Is there really a difference in Polish between large and big? If so, what is the word for large, as in a large dog.
No, "large" is generally accepted for "big" in this course. This sentence for some reason had "huge" instead, which is too much in my opinion. Added "large".
Why 'This boy dislikes big dogs' is wrong?
"dislike" is not a common report, few people use it. But yeah, it suits the sentence, so okay, added.
can someone please explain the difference between:
chłopczyk, chłopak, chłopiec
chłopczyk < chłopiec < chłopak.
It's an age thing. "chłopczyk" is the youngest, then "chłopiec", then "chłopak". There are no clear age borders, it's very subjective.