I would imagine, like any board, the material, number of layers, spacing, controlled impedance.... will all be part of it. Yields from the board house may go into the toilet.
Four of them come to mine. One very old using a mix of surface mount and through hole on FR4, 4 layer. I would guess was around 16" X 14". I am looking at one that is a little newer on Kapton, all surface mount which is 10" X 15". This one has no board stiffeners as it's on an aluminum substrate. We made some larger ones from Arlon. The worst was a power supply that I bet was close to that 16X14 board that required several stiffeners. We later changed that design to a board maybe 6X6.
I would talk with your board houses and see what they have to say. You may be shopping around a bit.