A nonce should be decided on the basis of the difficulty conditions you want to satisfy. Firstly, set the difficulty conditions of the blockchain and then decide on the nonce. Optimally, a nonce shouldn't be too big to take unnecessary large space. When developing your blockchain make sure you don't create impossible to satisfy conditions.
Sometimes, there may be situations where even a 64-bit value is insufficiently large to get a satisfactory hash output, so make sure there's other things that can be adjusted, re-ordered, or incremented as necessary.
You can consider using 32-bit value for your blockchain because the nonce is tried sequentially so you won't need bigger nonce value.