Once you've decided your business model is ready for estimates, find Estimate on the side bar of your canvas, as seen here:
Next, select a customer segment post-it and enter your market size estimate before hitting the Submit button.
Once you've saved your estimate, a blue bubble will appear on the top right corner of your selected post-it. The size of the bubble will be proportionally larger or smaller based on your actual numerical input.
Note: You must hit Submit to save your estimate. To erase your estimate, change the number to 0 and hit submit. This will remove the blue bubble from the post-it.