Shot and edited at 60fps, how can I downsample?

I think I made a rookie mistake. I shot and edited my video at 60fps, but now that I'm trying to export it for youtube I find I have a huge file size (even after compression).

What's my best option to reduce the file size? I've read that I could use 24fps, but I don't know how to downsample all my edits (creating a new 24fps project and copying and pasting the storyline doesn't work). What else can I try?

Thanks

You can do it in Compressor easily, but to stick inside of FCPX, simply create a new Project, custom settings, 24fps, progressive. Go to the original timeline, Command-A (select all), Command-C (copy). Then back in your new 24p timeline, Command-V (paste). Export.

It didn't work. When I paste my new project goes from 24p to 60p.

Put in something that is 23.98 on the timeline first. Then copy paste. Then delete the other thing. That's the simplest way.

As Ben noted, make the project using custom settings, (custom settings, 24fps, progressive.) not "set based on first clip". Or if you leave it at automatic, as Darren says, cut a clip with the format you'd like into the timeline first, (now the format is set) delete it then paste your material into the new project timeline.