Leafy greens are 2 cups raw and 1 cup cooked as a serving size according to the MyPlate.gov website.I’d consider canned to be cooked. I’m sorry but I don’t know the gram weight on that (it would likely vary quite a bit depending on water content).
Drain spinach, use a salad spinner if possible, weight the entire can’s worth of spinach, get total gram weight, divide by number of servings in entire can, you’ve got your serving size.
Maybe don’t use a salad spinner, IDK. But that’s the basic process