New controller
 

I have a 27mhz Microsizer. I have a Niko RC. It's a big one, not a tiny rc. It's also 27mhz. The controller has proportional steering. Will it work on my micro sizer? I don't want to mess up my car, so i didn't try yet. If it does work, will it have proportional steering?

If it does work it won't have proprtional steering and it won't mess up your car in anyway either.

Most likely it won't work and no you won't have proportional steering even if it did work. Proportional steering requires something far more complex than these little cars to work.