I agree with what RCA is telling.

The issue your wife is having is partly her problem, partly the boards problem.

What you are learning you actually want a BIG slow board that is very heavy when you pick it up. This is why the CWB, Liquid Force, and a few other boards are so popular.

They are very UNresponsive to your movements. This allows new riders to be jittery, and make movement mistakes and the board won't respond to them so fast.

Getting a Smaller board isn't always the better answer, your wife should be able to ride your board just as you do.

When she gets up, if the board is unstable its normally due to your feet not being wide enough apart, having your knees bent too much, or standing unlevel on the board.

She needs to stand pretty centered on the board, this way the board isn't trying to carve, nor is she riding the brakes or the gas.

Another thing that helps me teach new people is pull them up slower then a normal rider, also don't go right to 10mph, she is light enough the board will come up and let her ride at 7mph then when she stabilizes speed up to 10.

A Skim board is technically harder to ride because it only has 1 small tracking fin, but some people <smaller lighter> will like it better because its less buoyant, and has more drag in the water making it feel slower then a traditional surf board that is more buoyant.