How to say "One of these" and three other mini phrases?
No, it's not stupid at all. I think they can understand what you are saying. 

If you want only one, you could say 这个来一个 ,  我要一个这个, or 给我称一个这个, pointing to the fruits you want to buy. And you could add 只要一个 to emphasize that you need only one.

If you want a smaller/bigger one, you could say 我想要个小/大一点的。 or 能换个小/大一点的吗？

这里有...吗？ or 你们有...吗？ are both okay.