Heck yeah I have doubts every time I buy forms. I’ve had pretty good luck though – the breast plate was the only time I missed badly on sizing.
The only way to know for sure you’re getting the right size is to get fitted at a store. For most of us that’s not an option.
The next best option is to contact the seller and ask a lot of questions. I’ve had good luck with The Breastform Store and Glamour Boutique. They have well trained staff with lots of experience helping customers find the right forms. It’s likely they will know specific bras that work well with a particular form. It will help if you know your bra size, your underbust measurement, and the specifics of your current forms.
You could also buy from an Amazon seller that accepts returns. Tara Jeane commented in this thread about Kabeilin breast plates having a no-ask 30 day return policy. If it doesn’t fit you can exchange it for another size. Of course, you have to be okay with participating in a “used” marketplace for a very personal item.
I wouldn’t be too concerned about a D cup unless you’re really skinny. I’m 5’11” and 200+ lbs. I wear a 40 DD/E or a 42 D and think they’re the perfect size.