There is a very big difference between vaginal and anal penetration. My wife likes a little larger head with a slightly smaller shaft. This type of dildo stimulates her a-spot and g-spot without making her vaginal entrance feel too stretched.
Anally on the other hand, for us at least, everything has to be smaller. A tapered head eases entry to a little larger shaft that provides a nice stretch along the shaft. In fact my favorite has a wave pattern that gives a stretch and release feeling when thrusting.