About how big a share of the global economy is the U. S. economy?

22 percent.

Explanation:

The economy of the USA nowadays still remains a huge factor and is at the top of the pack, together with China. With little over 1/5 of the world economy they are an economic giant.

On the other side, USA even though still has its position as an economic super power, it must be noticed that it has seen around 50 percent drop in the total world economic participation since the 1960's. This is due to the fact that China has become an economic world power in meantime, as well the rise of some economies that were destroyed after WWII like the German and Japanese economies, which gradually lower the global economic influence of the USA.