I wouldn't say you NEED to have a set of 4. I ran 2 snows on the front of both my lumina, The grand prix, and the bonneville for the last 4 years. No problems. One thing that should be noted is that I had good rear tires too(60% or better tread life, the more the better).
Assuming your rear tires are good, your rear end won't go bye bye.
Heck, on my regal I just had a good set of regular tires that were fairly new and didn't have a problem at all.
As for size, it helps to match the sizes. Not sure what the difference in size translates to for you, but it shouldn't be that big of a deal.
Remember though, snow tires don't allow you to go faster in the winter, they allow you to be safer when driving reasonable for the weather conditions.
2007 Saturn Aura XR | 1970 Chevy Blazer 4x4 | 1970 GMC 3/4 Suburban 4x4