I eat once a day. a large meal. is that healthy?

Not necessarily: Better to eat three small meals to regulate the blood sugar levels you are getting and to allow more regular digestion. If you have been doing this for a while without deleterious results, who knows?

Stomach Storage: The stomach is a storage organ and yours must be large enough to contain a whole day's food at one time. This is fine as long as it works for you. Warning signs would be: sleepiness or fatigue 2 hours after your meal, heartburn or reflux when you lie down, a sour, bitter taste in your mouth when you wake up (reflux) cavities in your back teeth (acid erosion) or weight gain.