350 kilobytes

in theory it's possible to make them up to 4 megabytes but my javascript screws up when you get much bigger than 350 kilobytes

and nodes won't relay transactions that are bigger than 400 kb anyway, though they will accept them if they see them in a block

When i try to upload a file higher than 120 kb or something it gives me a pop up saying it has a limit. Are you sure it's 350 kilobytes?

It has a soft limit (120kb) and a hard limit (350kb), you can override the soft limit by clicking through the prompt, which tells you to click ok to ignore the warning. But you can't override the hard limit except by modifying the code.