They sell rat balls as well as ferret balls. Rat balls aren't really suitable for rats but are big enough for an adult syrian whilst not being too big for them to comfortably use. The ferret ball does sound like it may be too big for her. If the ferret ball seems too big and you don't mind spending a bit more money then you could have a look for one of those. They have them here:
https://www.amazon.co.uk/Savic-Runne...words=rat+ball