That depends on how you define battleship. And a 5KTd ship will NEVER be M6 under Bk2.
_ Maximum Hull Size by TL and Rating under Bk2
_ TL _ Drv ___ R1 ___ R2 ___ R3 ___ R4 ___ R5 ___ R6
__ 9 _ A-D __ 800 __ 400 __ 200 __ 200 __ 100 __ 100
_ 10 _ A-H _ 1000 __ 800 __ 400 __ 400 __ 200 __ 200
_ 11 _ A-K _ 2000 _ 1000 __ 600 __ 400 __ 400 __ 200
_ 12 _ A-N _ 2000 _ 1000 __ 800 __ 600 __ 400 __ 400
_ 13 _ A-Q _ 3000 _ 1000 _ 1000 __ 600 __ 600 __ 400
_ 14 _ A-U _ 3000 _ 1000 _ 1000 __ 800 __ 600 __ 600
_ 15 _ A-Z _ 5000 _ 5000 _ 4000 _ 3000 _ 2000 _ 2000
That's a very useful table there, Aramis. By the way, when you consider several 'standard designs' improbable, which ones did you have in mind?
A Battlecarrier with loads of fighters is gonna be SLOW under Bk2, and dominate not by direct combat, but by sending fighters out. Any direct combat warship will be outgunned by the fighter carriers.
Later I'll try and post my concept of a battleship or cruiser using Book 2, it certainly won't be 5,000 tons since, as you've identified, its drives will be comparable to a cruise liner
