How big is your yard? That's the ruling factor. If you have 15 feet of yard and enough width for a 6 foot yard ladder, #6 or even #8 turnouts will be ideal since they are more protoypical and you don't have to worry about car length restrictions when you're switching. If you only have enough room for a three or four track yard with four five 40' cars per track, you'll have to use #4 turnouts to fit in the ladder track. That will limit the length of car you can switch into your yard without derailemts. It's all about space and, the bigger the space you have, the bigger the trunout you should use.
I also have a four track yard and am stuck with #4's as a result. We can't change the laws of physics and create space out of thin air. I will say that laying out a yard with #4 switches takes a good bit of care and testing in track laying. If you get everything straight and smooth and get all the points filed so the cars go through the switch smoothly, you can move some pretty good size equipment through a #4 at slow speeds. I have an 80' combine that regularly switches in and out of the yard with no problem as long as I keep the speed down. Now, if you just slapdash your yard together, I can guarantee you that you won't be able to get a 40' boxcar in and out.
Big Boy? #4 turnouts? Don't do this. Please please don't do this.