A lot of the dildo buying guides I've read recommend using your finger(s) to get an idea of the diameter and/or girth. Usually 1 or 2 fingers is a good starting point, so you wrap a measuring tape around those fingers and go with that number. Length is less of an issue since you only insert what you're comfortable with.
I'd like to warn you to watch out for the material you choose. Materials like glass and metal have no give, so I avoid them, but even in sticking with silicone there's such a variety of firmness that I've found that toys with the exact same measurements can still feel different in use.
My vaginal opening is stubborn, but I like feeling full (not stretched!) and G-Spot stimulation. Here are some toys that I found easy to manage (as far as girth goes):
I use a thick water-based lube with my toys, and I only insert them after making sure I'm aroused, to keep from injuring myself. I wish you and your loved one luck and pleasure in finding what works for each of you