That (the bolded sentence above) is a rather sweeping statement. I can easily think of successful point guards in the WNBA who are not significantly bigger than Crystal, such as the Sun's Courtney Williams (remember her from USF?) and Chicago's Courtney Vandersloot. Both play at or near all-star level in the W. There are probably others that I am forgetting.
Maybe you were referring only to defense, but as a Sun season ticket holder I have not noticed that Courtney Williams has any major defensive issues at the WNBA level either. I don't think that either Crystal or Courtney (or their respective coaches) measures their defensive success mainly by the number of steals that they get. It's more about whether they are able to stop penetration, alter shots, interfere with their opponent's court vision, and get a few rebounds against their opposing guard. So far this year, Crystal has been able to do all of those things. Nevertheless, I agree with you that it probably will work better if Crystal guards Minyon Moore and either Aubrey or Megan (or possibly Anna) guards Sabrina. Or, as others have suggested, maybe all three of them will get their turn. The goal isn't to shut Sabrina down, but to hold her to her scoring average and frustrate her court vision and playmaking.
This whole thread seems to me to have a pessimistic bias because it fails to consider the defensive problems that UConn will create for Oregon. Will they play a zone like they did in the game a few years ago, which they lost by nearly 40 points? I doubt it. But I don't think they have seen a guard pair like Crystal and Christyn yet this year. Sabrina will have to guard one of them, and she is not nearly as quick as either of them. I think that is going to be a problem for the Ducks.
In the front court, there is a similar issue. If you assume that Sabally is going to guard Olivia, then Hebard and Boley will have to guard Megan and Anna, and I think both of them are at a disadvantage in doing so, no matter which way they split the assignment. It's true that Megan, Anna, and Christyn (as well as Crystal) will have to hit their jump shots at their average rate (which they all failed to do against both Baylor and Tennessee, for reasons which remain unclear but probably relate to nerves), but I don't think Oregon will be able to stop any of these players from getting makeable shots. And who does Oregon have that is going to stop Aubrey if she plays like she did against Tennessee?
In short, I don't think that UConn's defensive dilemmas are any more vexing than Oregon's.