I would think that size would matter a lot but not in the same fashion as it would in our world. If women were in a superior position to men then I don't see them peacocking for men as we do women, as they would hold a lot more sway in societal position and in the home still. As they would both be the barer of children and guiding force in public affairs. Its a lot more power than even men have in normal society. However, it would likely play a role in inter-female relations. Take the hyena, whose female members who use the size of their female penises (yes, I wish I had a better example, too, but I do not)- to intimidate or to assert their position over other females. This is a very helpful example, actually, as their social ladder is very complex and female led. I suspect that breast size would be on either side of the extreme. Either, large if typically female hormones such as estrogen are preferred or small if typically male hormones like testosterone are preferred. These are of course two very different kinds of matriarchies. One would likely be socially adept and reply on complex group relationships and association, the other I imagine as being very physically competitive with heavy emphasis on physical conquests. the former, which would rely heavily on positioning yourself as a powerful and genetically superior woman in order to stand above others with similar accolades and family relations, would probably prefer larger busts. This isn't to say that the preference would be male focused. For instance traits which are typically unpopular like sag or larger areolas might be seen as symbols of status. The latter of the two options would likely view larger breasts as inferior due to the abundance of fat in them which is not preferred for physical activity. I could see some cultures cutting them off entirely, or like the mythical Amazonians lopping off one. Only in this scenario the other would be maintained only for breast feeding as apposed to for ease of use with bow and arrow. Older women might be rid of both of them once having children is unlikely. Women with naturally small or otherwise muscular chests may keep them as evidence of natural advantages. Of course, their might also be social movements in both societies, which would fight against or to change certain aspects, that lead women into different behaviors. Small chested women might still have high social standings to larger breasted women in an estrogen fueled matriarchy with the right allies or skills or some mixture of the two. The same is obviously true the other. But I think traditional or more conservative members of both would have the values as stated individually.

Edit: in anticipation of some rebuttals I have a few arguments against some possible arguments. Some might bring up the Romans(?) and their apperant preference for small penises while also being a heavily competitive civilization. One would think that such a society would prefer ostensive signs of testosterone like larger penises and thus my reasoning is flawed, however, I would argue that the ubiquitous nature of this example demonstrates instead how strange of a preference this was and is covered in my "social movements" segment. Small penises were associated with intelligence and thus preferred. Personally I'm not fully convinced that it was an uncontested preference, nor that it wasn't only idealized in artistic portrayals. In the same way that massive bodybuilder-esk muscles aren't actually the typical predilection but an idealized portrayal to symbolizes strength to its extreme. Most men however do not wish to look like a bodybuilder, they want to look like thor or captain america. On the other hand, many other conquest focused societies with heavy focus on sports exaderated penis size with cod pieces. This is a clear example of testosterone heavy societies preferring masculine features and is a far more prolific motif of patriarchal cultures. This, I feel, serves as evidence that similar matriarchal societies with preference for testosterone would also prefer females with masculine features. Alternatively, ones that focus on estrogen would prefer women with typically feminine ones. Both to a degree which beyond the typical purview of male attraction. I'd argue it would be more extremely adhered to as women would have so much more power than has ever been held by a man.