There is no magnitude. They are different things, with different psychological roots, and different societal outcomes. There only relationship they have is that racism may involve prejudice (it also may not, although as I mentioned earlier that's a really hard stance to defend today).
https://en.oxforddictionaries.com/definition/racism
https://en.oxforddictionaries.com/definition/prejudicePrejudice, discrimination, or antagonism directed against someone of a different race based on the belief that one's own race is superior.
Preconceived opinion that is not based on reason or actual experience.