Here's a picture showing a variety of sizes with a ruler for scale. The Velella on the left is the more typical size that you "expect" to encounter on a beach.
Below is the same picture, but it's a close-up of the left side of the ruler:
Now you can see the smallest of the Velella, just a couple of millimeters long. They looked like this for almost the entire length of the beach that we walked, about 3/4 mile.
I know they were probably variable along that stretch of beach. But if we used this picture just to come up with some sort of estimate — then there were 25 tiny Velella/10 cm or 2.5/cm, and there are 160,934 cm in a mile, so there would have been ~400,000 tiny Velella washed up on a mile-long section of Salmon Creek Beach tonight. No wonder I kept saying, "Wow" as we walked along next to the receding waves.