Hello!
Sorry for the time we took to answer :/
I've worked on implementing the maxQueueSize feature :)
It should be available in our next release! But in the meantime, you can do it yourself like this:
Also, make sure you add the ToastManager on the scene first, so you can configure the maxQueueSize in the inspector. You can do that via MaterialUI -> Managers -> ToastManager.
~ Yohan