I have giants in my sorority and they don't bother the girl who's about an inch long at all.
If you'd like to try, and assuming your tank has adequate cover, it doesn't matter if they're "used to" their home as bettas tend to explore and stake out a territory quickly. If they've been together more than two days or so they probably know who's alpha, where they are going to rest, etc.
Just overfeed for a day or two, float everyone so they can see each other, and then release and observe if you want to be conservative! You could also just rearrange some decor and after acclimating add the bigger girl and observe. There might be a few nipped fins, but if there are no signs of extreme aggression and they are calm within a few hours it should work out. They might just re-establish territories and move on with life! IMO size is not important, unless say maybe your whole tank was giant females and there was ONE tiny gal...
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