There is a valid argument for us building the stadium with a 60,000 capacity based on stadium expansion or moves for other Premier League clubs. I had a look at other clubs that have moved and will be/have been in the top flight recently: Arsenal, Brighton, Leicester, Man City, Middlesbrough, Newcastle, Stoke, Southampton, Sunderland and West Ham.
Of those clubs the average attendance increase from their final season at the old ground to the first in their new/expanded stadium was 13,419. There were 2 anomalies: Stoke and West Ham. Stoke only saw an increase from 12,698 to 15,025, despite a record attendance of 51,380 at their peak, whilst West Ham had an increase from 34,910 to 56,971, despite their record attendance only ever being 42,322. Taking those 2 clubs out the average increase is still 13,725. Leicester were the only other club who had an increase less than 10,000 (9,396).
An important point to consider about Stoke is that they were a second division side when they moved and were immediately relegated to the 3rd tier which explains such a poor increase. With all due respect to Stoke, their fanbase is not comparable to ours. However, I found them a good example of the bigger increase in crowds that can be expected when a club achieves relative success. After achieving top-flight stability with 3 9th placed finishes their best average since the move has been 27,534. 2 other clubs that had a big rise in attendances following their initial move were Man City and Sunderland. Man City averaged 46,834 in their first season, but now after winning a few titles and being a CL club they are up to 54,041. Sunderland only averaged 33,492 after their move from Roker Park, but with a couple of 7th placed top flight finishes, they achieved a highest average in the new Stadium of 46,791. The average increase of these 3 clubs after them showing signs of progress was 20,079.
Therefore, if we follow a similar pattern to the rest and are in line with the average increase we would increase by around just under 14,000 putting us at a 53,000 Capacity with no room for manoeuvre. A 55,000 seater would therefore only accommodate for 2,000 extra fans and Everton would have to weigh up whether it would be worth the risk of turning another 5,000+ fans away and missing out on the financial gain that might bring. If we achieve some relative success with some trophy wins or top 4 finishes which has to be the aim then we could be looking at an increase in line with the average of Stoke, Sunderland and Man City which would be around 20,000. That would bring us to the 60,000 Capacity and shows what our potential could be.