For investors trying to pick the next fundamental, technological, breakthrough investment, it can be quite the dangerous task if approached foolhardily. The 3-D printing arena has surely been heating up, and if they are thinking of investing in companies like 3D Systems (NYSE:DDD), Stratasys (NASDAQ:SSYS), or ExOne (NASDAQ:XONE), then investors need to answer one very serious question: How much should be allocated toward these stocks? In the video below Motley Fool analysts Blake Bos and Isaac Pino discuss 3-D printing and how investors should determine what percentage of their portfolios should be allocated to companies in this booming sector.