I remember the first workshop I signed up for that required I bring a canvas that was at least 18x24. It was a portrait workshop with Michael Shane Neal. First of all, portrait??? Me??? That was already a crazy idea, because I had not been painting long at all and basically, well, was pretty awful at it. But it was the size of the canvas that seemed the most daunting to me. I had been painting so much smaller, 8x10, 9x12, 11x14... 16x20 at the most, and even the thought of jumping up to an 18x24 or 20x24 frightened me.
It was many years later that my husband built me an extremely large easel. Suddenly, even truly large canvases did not seem so large any more. Previously, when I put a larger canvas on my small easel, it had seemed overwhelming to me, but on the large easel the same size canvas now felt small. I began by painting 36x48. They felt tiny, so I tried 40x60. They were no big deal either. The latest was a 48x72 (shown below). Really, not so much different than painting small.
So here's your pep-talk for the day. Repeat after me... "I can do this. I am in control. It's really no big deal." Now click your heels three times and DO IT!