Do you really believe that a 5-gallon pot is to large for a seedling and might be detrimental to the seedling? I might be wrong but I just have to believe that the planet earth is at least slightly larger than a 5-gallon pot is, and seedlings do just fine growing in planet earth.
I never use anything smaller than 5-gallon pots, and more times than not I use 7-gallon pots, and I always start out my seedlings in whatever size pot I will use for a particular crop. I never re-pot and I have never experienced any downside from using larger sized pots from seedling to harvest. If anything, I have seen healthier plants, better and faster growing plants and I do not risk damage that can occur during re-potting.