People who grow up almost anywhere else on earth can also tell how big something is based on their experience with metric. That's not something inherently based on the imperial system. The same way you go "oh that's about 3 feet", we go "oh that's about 2 meters".

And of course, switching systems overnight is insane, people are used to imperial, you're right. But at the very least do what Britain did, and have both systems in parallel at the same time, everywhere. And in time, people would get used to metric too.