54 is a fine size. I can ride the 52 everywhere and I'm 155ish lbs.
As for the minimal amount or camber. The industry basically allows you to call anything zero +/- like 2-3mm of camber/rocker. It takes so little weight to level out that much camber it basically isn't there.
The reason for it is the way a snowboard is pressed. With there being a heat blanket on the top and bottom, depending on initial resin temp, ambient temp in the factory, the difference in temperature between the blankets, the overall temperature the press gets too, the small variations in wood grain... those all effect the level of camber in a deck.
I already have a Westmark for park style riding. Thinking of getting a longer Park Flat to be more of a do-it-all board for those days when I can't or don't want to decide between all mountain and park. I was thinking a longer, hence stiffer, Park Flat would be such a board. I'm 180-185 lbs. Is 158 the longest that the Part Flat goes? How stable will this board be for charging groomers and variable snow conditions at my weight? Skill level - progressing intermediate.