How many jumbo marsh mellows are equal to 16 oz.?!

Look at the bag - it should have the net weight written on it. Net weight is the weight of the contents, not the weight of the bag - i.e. what you want to know. So you don't need a scale.

I suggest that you buy a scale in any event - it's almost impossible to cook anything without one, and they aren't expensive.

They normally say 40 mini marshmallows is 16 oz. I don't know if that's true or not, but anyways, 4 mini marshmallows are approximately equal to 1 jumbo marshmallow.

You can also look at the bag - 16 oz. is 16 oz. and I'm pretty sure the bags contain 16 oz. of marshmallow regardless of the actual size or shape of the marshmallows inside.

Place them in a measuring cup. 16 oz. equals two cups.