Size of "a" vector

From Recurrent Neural Network Model Video:

I know the size of the X vector depends of the size of your dictionary, as it’s a one-hot index into the dictionary.
What would the size of the A vector be?
Is that up to the designer of the RNN?

It’s the number of previous states you want to remember. It’s a design choice.

You’ll see it in action in the Week 1 programming assignments.