UPDATE: Here are some signs from the marches around the world. This sign describes me and a lot of the people for whom this may be one of the few or only rally they ever attended.
I just returned from the Cleveland March for Science. I spent my time at the pre-march events in Public Square and waited for the talks but came home when the march proper started. I have little experience with marches and rallies so have no means of comparison and estimating numbers. All I can say is that it exceeded my own expectations. It took quite a long time once the rally ended for the crowd to leave the square on the march, which is a sign of how big the crowd was.