China's GDP growth has remained above 6% since Trump was elected in November 2016. While Chinese economic growth has started to slow, that trend was already underway before Trump was elected.
In 2017, China's GDP was $12.238 trillion, according to the World Bank. While current estimates are hard to come by, China's economy grew by 6.6% in 2018, which would put the size of the economy just over $13 trillion. If China were to lose $15 to $20 trillion it would completely wipe the country off the face of the world's economic map. The bigger point Trump seems to be making here is that his trade policies are hurting China's economy. That at least does carry a kernel of truth. (As economists have noted, however, the US economy is harmed by these tariffs as well.)