Currently it is not possible to determine TBQueue capacity, it has to be stored outside.

Can we add sizeTBQueue to return the max number of the elements that it can hold?

The implementation is trivial, given that it is fixed it doesn't even need to be in STM monad unless I am missing something:

sizeTBQueue :: TBQueue a -> Natural
sizeTBQueue (TBQueue _ _ _ _ size) = size