I suppose it all depends on the look you are going for. Also, are you just building a pair for the entry, or are you using them around the entire fence? I guess my only real concern about doing 12" wide and 6' tall would be make sure you check the stability. Although you'll probably have your columns all interconnected, you want to make sure the base is wide enough to keep the columns from being tipped over or blown over. Also, because the fence sections I'm building are only about 40" in height, I thought the columns would appear more in proportion at around 4 feet. But I suppose taller ones could lend a neat effect as well. It would probably be REALLY cool to have entrance columns taller than the rest... hmmm, I might have to make a new set of taller matching ones just for the entry for next year! Keep me posted on what you do and how it goes... I'm really interested to see what you come up with!