Since you said the 20mm oval fell right out, personally I would probably order the next size up and one above that depending on cost. I imagine you could always resell the eyes that don’t work for you too if you couldn’t use them in the future.
I just used the oval glass eyes from BB in my scarlet kit and ordered the size the kit called for and a size up. Both worked, the eyes that were the size required by the kit weren’t as snug tho. I ended up putting a little piece of torn sponge in the back to keep them securely in the direction I had placed them. It was tricky gluing the flap with a piece of sponge in between the eye and the flap, but I used my hemostat to help hold the flap closed while I applied the glue and while it dried.