Miracle potion, it is.
Picture this:
Him - ~6'8" tall, around 8.5" x 6"
Her - 5'0" tall
The only way that was fitting for extended periods of time was lube or a shoe horn, and I much prefer the former.
The only warning I have is make sure the guy is in some form of constant communication with you -- I've had the effect of bottoming out and even though it killed me pain-wise, I'm sure it's 10 times worse for her.. it does rule out some sexual positions, especially when there's a 2 foot difference in height.
Not that it doesn't have its advantages, too