Your maximum hand size is checked only during the cleanup step on your turn. If Nezahal's last ability is activated before your turn's end step, it will return before your next cleanup step and you'll have no maximum hand size.

Nezahal's triggered ability resolves before the noncreature spell that caused it to trigger.

Players can cast spells and activate abilities after Nezahal's triggered ability resolves but before the spell that caused it to trigger does. Notably, the card you draw may be able to counter that spell or may be discarded to activate Nezahal's last ability.

After Nezahal returns to the battlefield, it will be a new object with no connection to the creature that was exiled. It won't be in combat or have any additional abilities it may have had when it was exiled. Any +1/+1 counters on it or Auras attached to it are removed, and any Equipment will no longer be attached.