Economists have long used letters of the alphabet like V and U to describe economic recoveries. But the coronavirus downturn is so different from past recessions that economists are coming up with new shapes to describe the potential recovery. WSJ explains. Illustration: Jacob Reynolds WASHINGTON—U.S. government debt will exceed the size of the economy in the government’s 2021 fiscal year, a milestone not hit since World War II that has been brought into reach by a giant fiscal response to the coronavirus pandemic. The Congressional Budget Office said Wednesday that federal debt held by the public is projected to reach or exceed 100% of U.S. gross domestic product, the broadest measure of U.S. economic output, in the fiscal year that begins on Oct. 1. That would put the U.S. in the company of a handful…