Officially? No there is no way to set a limit on “share size” with the single bay My Cloud device.
Unofficially, who knows. There might be some sort of way since the My Cloud OS is a form of Linux. It is possible one of the other OS’s people are loading/experimenting with may support such a feature.
https://community.wd.com/t/clean-os-debian-openmediavault-and-other-firmwares/93714
Edit: A similar feature request was made in the Cloud Ideas subforum a while back:
