Please help me with snow tire size options?

[986 Boxster]

I don`t think much of the problem is the 255 tires at the back, it could be that the tires are worn or old or that the car is to light in the rear, etc. I run winter tires (245/35r18) in one of my cars that`s rwd drive and I have no problems with traction, unless I push the car. But like others have said, taller and wider tires are much better for the snow. 225s/45 or 50 or 235s/40/ or 45 for the rear and you can use the 225s also at the front if you want to go with a square set up.