I'd look at existing products in the same space and try to figure out if you want your size offerings to match them or if you want your product to be differentiated by size offerings. Go to a store and look at roughly who your larger-scale competitive landscape looks like.
As far as industry comparables, I have seen packaged popcorn products that target the same space as "sandwich and chips" lunch specials and they typically offer a weight around .5-.7 oz (maybe a bit heavier for sugar-glazed items, say 1-1.5 oz); 1 oz by weight will translate into a volume about 2.5 cups. Anything in that range (1/2 to 1 oz) can easily pass for a single serving.
Microwave popcorn packages appear to typically have 1.2-1.5 oz popcorn (4-5 cups volume after popping). Some people consider those single servings.
(I've done no particular investigation of the percentage of weight attributable to oil/fats/seasonings/sugar/salt, so your mileage may vary).
I would not be surprised if a large percentage of packaged foods are sized based on what packaging was available for what cost. One product (not popcorn) I previously sold with piece counts delineated by what packaging style was readily available to me for a reasonable price; I could fit 4 pieces in a small box, 8 in a large box, and then I had "bulk" pricing (25 units or something like that) for people who didn't care about the packaging; I varied the bulk packaging depending on what was available to me from PaperMart or U-Line at the time.