In this articleEtsy Sellers Market in Times Square celebrating Etsy's celebration going IPO at Nasdaq on April 16, 2015 in New York City.Activist investor Elliott Management has a "sizable" investment in e-commerce firm Etsy, which announced on Thursday that Elliott partner Marc Steinberg would join the company's board.The activist investor has built a roughly 13% position in the stock, a person familiar with the matter told CNBC.Etsy shares shot up more than 10% on the news. Elliott and Etsy have been meet ...