Translation:Please give me five watermelons.
It depends. If the first word is two syllables, those two syllables become 2nd tone; Eg. 保管好 băoguăn hăo becomes báoguán hăo.
If the first word is one syllable, the first syllable of the second word become 2nd tone; Eg. 老保管 lăo băoguăn becomes lăo báoguăn. I think in this case it's the first one.
西 (xī) = west
瓜(guā) = melon/squash/gourd
西瓜 (xīguā) = watermelon
Squash and gourd are kinds (families) of fruit among which:
黄(yellow)瓜 = cucumber
青(green)瓜 = cucumber
南(south)瓜 = pumpkin
木(tree)瓜 = papaya
苦(bitter)瓜 = bitter melon
The native range of the watermelon extends from north and west africa to india, so it was introduced to china from the west. The pumpkin is introduced to china from the south.
By definition no. Here's the list of the nouns with 只 as measure word: https://en.m.wiktionary.org/wiki/Category:Chinese_nouns_classified_by_%E9%9A%BB/%E5%8F%AA
not all watermelons are big. Why not five? And if big, they can buy for many people. Also, maybe the reason is here, maybe watermelon are also daily clothes : https://kotaku.com/these-are-chinas-watermelon-kids-1076489968