I generally like a 2 (H) x 1 (W@base) ratio ... but a 5' to 6' base would probably be optimal. If you have some sag, the 5' may look a bit narrow. If you have no sag, the 5' could look great.
I'm not a big fan of the fat wide trees, but some people prefer them. Going with a base diameter which optimally fits your light string lengths is another good way to go, so take you string size, maybe fold it ... then figure out whats best. For me that a 100ct set of incan. minis, folded is around 12' hypotenuse.
The is no right/wrong answer as long as its what you like best.