I can assure you they are 19s. I'm not a G35 expert, but I do know that my buddy got the upgraded 19s as part of a sport package.
As for tread depth, I don't think my buddy has a depth gauge. Does the pic give a sufficient indication? If anyone is serious, you are welcome to see these tires in person.
I would have used these myself, but my tire sizes (e46 M) are different. This is a great deal for lightly used tires. I should also add: no track days, no donuts, no qtr mile races,...just some average spirited driving on the GTA highways.