With a guess you would be using 5mm pitch with that size motor, so all you have to see is how small that pulley size will go and still fit your motor shaft with Key Etc, all the timing pulleys have a minimum belt bend radius, that is why they have a minimum pulley size
It also reflects on how many teeth are engaged the smaller you go the less teeth will be engaged, so if you don't need to go to a minimum size then it has benefits, of more engagement, what are you driving with it, do you need to have it geared down, or are you using it 1:1