The heart size increase
before the surgery is quite common in case of regurgitation. This kind of "remodeling" is basically an adaptation response. If a smaller fraction of blood "goes forward", the heart tries to pump a higher volume, so that still a sizable volume is pumped forward. And this only works so far. I suspect it's basically a run-away condition, where the enlargement causes more regurgitation problems. There is also attendant weakening of the heart, since it's easier to throw a portion of the blood in backward direction (to a lower-pressure area). So eventually one gets a heart failure, if not treated.
After the surgery there is "reverse remodeling". Once the problem is fixed the heart volume shrinks back to the normal size, since that's all that's needed (*). There is a fast phase of shrinking, with a significant size change within weeks. But there is also a slow phase, where further changes have been traced (statistically) for roughly a year (**).
I'd imagine that the individual response may vary, of course.
In my own case the size was enlarged before the surgery, and was classified as normal from echo done 3 months after.
(*) There are different cases with high-endurance (or olympic-level) athletes, which can get enlargements as an adaptation response to high volume of exercise. As well as regurgitation, etc.
(**) This is just my more or less naïve interpretation of publications I've seen, e.g. this one:
Time course of left atrial reverse remodelling after mitral valve surgery and the impact of left ventricular global longitudinal strain in patients with chronic severe mitral regurgitation
