Yet they labelled it 'battlecruiser', British humour at its finest.
I sometime wonders where was the limit? If they had build these things, other nations would have to respond. If this escalate into unrestricted building race, what kind of monsters would they build after? By the 1940s OTL modern battleships would likely be dwarfed by the size of the new ships now under construction.
Budget and dockyard size. No point in building a 100k UBERVESSEL if you can't dock it anywhere but in the deepeset part of the channel. 60 - 70k tons would probably be about the upper limit really.
That was the idea behind the Tillmans, to stop with the incrementalism and just get to the biggest practical ships. Their only limits were fitting into the Panama canal, IIRC.