Well, now that it's 11 chapters since the end of vol. 37, I think it's safe to assume that Vol. 38 is completed, and we can begin guessing about it will look.
I think the cover will be Purple, with Puffing Tom and Rocket man on the left and right sides, with the Aqua Laguna right behind them. The Trains' Main occupants faces will be above the trains (Lucci, Robin, Luffy and Paulie for example.)
As for Size, I think it will be 358-367. Sogekings arrival seemed a fitting way to end it. However, I HOPE it will be 358-368, seeing as how we'd get a bit more action packed into a volume, where the plot has progressed (relatively) slowly.