How to say "One of these" and three other mini phrases? I'm very glad I found this website. Just a few months ago I've started learning Mandarin Chinese by myself and right now I'm in Sichuan. I try applying my little knowledge in daily life, but it's not easy ...also of course because they don't speak "proper" Mandarin here, but their craaazy dialect instead ...I'm trying anyways :)
A few things I'd like to know but was not able to find in dictionaries / through Google:

Let's say there's a fruit vendor and I want to tell her/him that I'd like "one of these" ...so far I've said sth like "这个。。。一个“ - and felt completely stupid doing so. Please help me say it propberly!

Same situation, but I want a bigger/smaller fruit of what he/she's holding in their hand. sth like "a bigger/smaller one, please"

When at a shop, asking for a specific item - "Do you have...?"
Could I say 这里有。。。吗？ and/or does even the literal translation 你们有。。。吗？ make sense?

and finally: "I only speak a tiny bit Chinese."
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.