Ah, it's larger in the photo than real life
It's just wide enough for my #5½ plane + ¼" so it'll ride parallel readily enough. The slats are mostly around 11-12mm thick and they're being planed down to ⅜" (so about 9.5mm) thick each, and they're only around 1¾" wide. The real pain was the rough-cutting because resawing 8x1 ash boards by hand is not as much fun as it sounds.