A.
anwered by: canteau
The freewheel tooth count is going to depend on the rear hub threading and if the hub is threaded for a freewheel at all. Standard English (BSA) threading will work with freewheels with a minimum tooth count of 16t. If the hub has the smaller metric threading you can go as small as 13 or 14 teeth.
Front chainring size is going to be determined by the crank bolt pattern and the max size of the chainring that will clear the chain-stays on the frame.