The bigger the stadium, the more the extra seats tend to cost (structural requirements alone grow).
5k seats @ £330 is £1,650,000 per year. The stadium will cost in the region of half a billion pounds, so financially 5000 extra seats won't make a difference to club's finances. It would increase the risk that income is lower, though, because those seats will have to be paid whether someone sits on them or not. And this payment will go on for decades. Way beyond the current owners or their successors. Many of us will be long dead and still the club will be paying for the stadium.
Whether the stadium has 55k or 60k has absolutely no effect on the sponsorship deals. The only thing that matters in that case is how well the team does on the pitch. Two million extra per year will not help (and remember, this assumes every game is sold out) when you consider our revenue is over 150 million already, and will be, IIRC, about 180 million with the new TV deal. You can pay wages for a reserve with that (£40kpw).
The real money in the stadium comes from corporate boxes on matchdays unless the club raises ticket prices. I think that would have a detrimental effect, though. Overseas fans would be also good as they buy more merchandise but the number of Everton overseas fans is dwarfed by the bigger clubs. So this won't help either, unless the club first becomes very successful and stays like that for a decade at least.
To me this all "we want this big stadium" is more about wanting a bigger one than the neighbours which is just stupid. Stadium size should be based on facts and figures because that's the only way to actually build a stadium that benefits the club.