She herself will know where to put her luggages if you tell her you don't have any spaces for her luggages. Let her put all her luggages in your living room first. I am sure she will solve the problem herself as she does not want all her things to be seen in an open area.
My previous helper had her own room. At the time she left, she had 1 big suitcase, 2-3 small suitcases, 2 big 紅白藍. I didn't realize she got so many stuff. The problem is she was quite dirty (never cleaned her own room) and I doubted she brought in and kept cockroaches. So, try not to let them bring in a lot of things other than daily needs if possible. Some of those many be her friends.
If u give her much storage space, she will keep not only het own stuffs but also stuffs of her friends. That's true. We cannot be too generous in storage space.
My helper has been working in HK for 6 years. She doesn't have many stuffs - 5 plastic drawers + 1 travelling bag. She will send the unnecessary things and the children clothes/presents I gave her to Philippines twice a year.