The pros and cons of the size of the employee option pool are rather subtle and are discussed in detail at VentureHacks. I won't go into the details here; suffice it to say that during a fundraising, the option pool comes entirely out of the common shareholders' stake, but if you have to enlarge the option pool later, it is dilutive to both the common and preferred. Therefore common shareholders prefer as small as possible of an option pool, thinking that if they need to enlarge it later, the preferred shareholders should pay up, while the preferred shareholders prefer as large an option pool as possible, knowing that they don't have to pay with equity now, but if anything is left over unissued in the option pool, they will benefit.
To answer your questions:
If you are worried the CEO keeping a stake in the company, then you can add something to their employee contract that they get a certain percentage of the company. That way, the evaluations are worked our properly and the stock can be issued accordingly.