Please revise the official stratum documentation at https://slushpool.com/help/#!/manual/stratum-protocol. There are no such limitations, a nonce can be even larger. The fact that you're receiving different nonce sizes is not related to a particular NiceHash stratum endpoint. It is simply a fact that NiceHash is forwarding work from different hashing power orders, which are working on different target pools. Anyway, 17 bytes nonce is the largest (max) that you'll receive from NiceHash stratum and even at this size you still have 3 bytes for iteration. So there is no reason why not to accept this kind of work. Please fix this as soon as possible, since our users would really love to use your miner and are now not able to do so due to these limitations of your miner.

Thank you and keep up the good work!