Kim,
How exciting! I like the echo of the composition in the husband's portrait - regarding size, I think you should go for the same size portrait - after all the wall will become its own 'canvas' and I can't bear the asymmetry of the clock flanked by two different size portraits. I think the best way for you to consider it is to make a three value thumbnail sketch of the ENTIRE wall, to scale, including the clock, and both portraits.
Regarding the size, I think you should measure the husband's painted head size to see how closely it is to life size, and whether you feel comfortable painting in a scale that matches (in relation to % of life-size) Then I'd go with a like proportion of her head size. The enormous difference in their heights might make for a design problem that is difficult to overcome. I fyou decide to do a matching vertical composition, you might still be able to indicate their relative heights, perhaps running some wisteria or other foliage across the top like a trellis, in order to avoid the feeling that she is sinking off the bottom of the canvas.
I guess I would ( if it was my wall) prefer to see a proportionally scaled portrait of her in a horizontal format, same size canvas, than two vertical pieces of differing sizes. A horizontal composition would also allow you to place her head closer to the top of the canvas - and on a more comfortable eye-level for the viewer, than would be workable if she were standing in the same place.
I wonder why they didn't buy the original of the husband - just something to consider in your contract
And Double Diamond Ranchwear makes some terrific dress-western clothes!
Good luck,