It might be best not to try to use it.
It is informal. It is colloquial. (In fact I see that the American Heritage Dictionary actually calls it "slang.") It carries a number of meanings, some subtle. Don't worry, it's not a "bad word" or anything like that. It carries the ideas of "very big," "surprising," or "excessive." It is used in a humorous, droll, or joking way.
For example, someone in a business presentation would never say "we had a whopping big profit last year," because profits are regarded as serious, not humorous. Similarly, you would _probably_ not say "the Mississippi is a whopping big river" or "Asia is a whopping big continent" because these are not usually humorous topics.
On the other hand, if we had had a really unusual amount of snow--nothing dangerous, no power loss, heat stayed on--I might way "we got a whopping big snowstorm the other day."
If you were astonished at the size of neighbor's Newfoundland dog, you might say to a friend, "Wow! That's a whopping big dog."
And--US readers will say "of course"--the hamburger chain, Burger King, calls its signature hamburger "the Whopper" to convey the idea that it is huge, in an informal and joking way.