bike sizing

After mountain biking for a while, I’ve decided to look for a road bike to help with the climbs and maybe develop another hobby. I’ve looked at my local marketplace listings and there are a lot of good deals but almost all of them are size 56. I’m 5’9”. After reading around, it is definitely too big for me even though I am at the bottom of the scale for the size. Is it a terrible idea to get a size 56? There is also a ML Giant which is apparently 55-56 to which I was wondering if that could maybe fit me better. Should I just wait and look around for a 54?