Any posture is okay as long as it is good posture. This means that the spine is balanced on the sit bones (look them up if you don’t know where they are) and retains its natural curve in the lower back. The shoulders are relaxed. The head is well balanced on the spine, with the roof of the mouth parallel to the floor (that’s why you’re told to tuck in your chin).
Leg position can vary — different people have different levels of flexibility — as long as the knees are supported in a stable fashion. Even sitting in a chair is okay.
The eyes are open but cast down, at about a 45 degree angle to the floor.
The hands form an oval, left hand over right, thumbs touching and rounded. They should surround your tanjen (Chinese: tiantien; Japanese: hara), the strong center emphasized in Asian martial arts, located two finger widths below the belly button.
The following link from the Kwan Um School of Zen’s Dharma Mirror illustrates several sitting postures. DharmaMirrorSitting