Not in Japan, not very big. Crowded, even. But the food is definitely Japanese-inspired and the servings are decidedly big.
It was a bustling place and our waitress was incredible - fluidly moving around the crowded dining space and taking care of the other diners. The large open front window let fresh air in, which countered the tight conditions inside.
The miso ramen is delicious! A rich broth with tasty vegetables and lots of noodles. A very good deal and quite filling. My wife got the Hiyashi Chuka which had a light but flavorful broth and she opted for vegetable tempura as a topping which was so good! There was also a hefty portion of delicious strips of tofu mixed in. The soft-boiled eggs on both dishes were a surprise, but were cooked well. We ended up taking some home because it was so filling!
We both had "sake slush" drinks, which my wife found quite tasty. Careful, they're more potent than they look!
Definitely worth a try; when you're on the Main, drop in to Big in Japan!