This document from the council, but it doesn't read as if that's the decided capacity. More like a £300m, 50k stadium is the minimum being sought and anything in excess of that will require larger payments from Everton to the council. In other words, this is a working figure, which - I would have thought - will almost certainly rise.
Also, the document contains this, which sounds like good news...
"EFC intend to use the Stadium move to facilit ate a vital Legacy Project at Goodison Park, delivering health, education, affordable housing and public spaces for the local community which is likely to stimulate further investment in the L4 area and will create social, environmental and economic benefits. Details of this will emerge from EFC in due course."
http://councillors.liverpool.gov.uk/documents/s210377/Invest to Earn - Everton FC New Stadium - Deal Terms RTC45.pdf