I know some size adjectives like 小さい (small), 大きい (big), and 並 (normal). Does anyone have a list of other size adjectives: something that could describe things ranging from micro things like Legos to massive things like sky scrapers? My friend said that I can modify the three adjectives. Can you help me understand how that works?
There is something called "personal watermelon" in American supermarket term, which can get up to 2.2kg and can serve two people: size comparison (the top one), close up. I don't want to call it a 西瓜. Would 小さい西瓜 (small watermelon) work? I feel that using that combination may make people think it is so small only a few scoops and you are done. -- Answered in comments: 小さい西瓜 should work.
For tomatoes, there is プチトマト as opposed to ordinary トマト. For corns, there is ベビーコーン as opposed to とうもろこし. For cabbages, there is ミニキャベツ as opposed to キャベツ. Since the counterpart for watermelon is not popular in Japan (as well as in most countries), there is no word for personal watermelons that you mention. It you want to create a new word, you might want to try these that are along the same line: ミニ西瓜, プチ西瓜, ベビー西瓜.