perhaps not as important,
Perhaps is the key word here, as we have not been given any information about the OP's parents, only that they are looking for a DA revolver of ".38+" caliber for "home defense".
Most of the suggestions so far are for medium frame size guns, with a couple suggesting large frame, and mentions of small frame guns as well.
Medium frame size is the way to go, since we know nothing about the people, other than they are the OP's parents.
We assume they are not "gunpeople? or experienced (if they were, why would the son be asking for recommendations for them?)
Size does matter, not just the size & weight of the gun, but also the size of the users. A large frame heavy pistol is great for many things. It is the best choice for some things. BUT if Mom is a small lady its not the most suitable thing for her to use.
And the same thing applies to the smaller revolvers in reverse. They might be light enough to be easily held, with grips allowing a better hold, but most people find them more difficult to shoot as accurately as larger pistols, and the recoil of even .38 Spl defensive loads may be too much for the inexperienced users.
And that is another point to consider. It has to be a gun that not only is physically suitable, but the recoil has to be within their tolerance range, or they will not shoot it more than once.
Now, this might be acceptable to them, they might fire it once (making sure it works) then put it in a drawer and possibly never handle it again, confident that it is there if needed, and that they could use it well enough at across the room ranges.
Here's an example, of how fit matters. My mother was a small woman. 4'10" (and a half! damnit!!
) wore a size 3.5 ring, most she ever weighed in her life was 109lbs. Grew up a farm girl, and was a very determined individual, besides. She dispatched numerous varmints/pest with her single shot JC Higgens .22LR rifle. She knew how to shoot, was a good shot, and sometimes even shot recreationally.
Dad had a stock S&W Highway Patrolman, 6" (N frame) .357, with the original factory small grips. Mom could shoot it accurately, SA, from a rested position. (like sitting with arms braced on knees). She could not shoot it accurately off hand, the gun was simply too heavy for her to hold steady. She did not shoot it DA, the trigger reach and pull weight was too much for her to manage with her small hands.
Dad got an Ortgies .25acp, thinking the small size would be better for her to handle. It wasn't. She couldn't rack the slide, due to the tiny gripping area of the slide and the strong spring were more than she could work.
The gun SHE chose was a Ruger Super Bearcat. Size/weight was about perfect for her. She could work the little SA gun quite well, and though it was "only" a .22LR, it was HER gun, and she worked it and got rather good. Good enough to get the local nickname "Annie Oakley".
Point here is, that what needs to be the top consideration for choices is what the smallest, weakest user can manage. Too much gun is as bad, or worse than not enough.
Take Mom & Dad to a gun show or a shop, where they can BOTH handle different examples. If there's one with a range available, go there hand have them both shoot some different things.
Let THEM choose (with guidance) what suits them best. Your and my opinions of what is "best" might not be best, for them. Guide them, give them some hands on if possible, and let them make their choice.
Don't let them be swayed by an enthusiastic salesman pushing what he needs to sell as "the best thing". It might be, but often isn't. Everyone's situation is different in different ways.
Good Luck, there is something out there that they can use, and is suitable. Something less than the "best" choice, that they can use, understand and have confidence in is better than the "best" thing (in someone else's opinion) that they can't use well.
Good Luck!
and let us know what they settle on and how we may help, in the future.