Here are some examples of "Is there ___?":

Is there a day when we could meet and discuss this in depth?

Is there anyone I can talk to about the status of my application?

Is there something you wanted to tell me?

Use this kind of sentence when you want to ask about whether a certain kind of thing exists.

Use "Is there ___?" in situations where you need to be polite. When you ask a simple question like:

What size are you looking for?

...the question assumes that the listener is looking for a size. But maybe he isn't. Maybe he's looking for a certain style instead of a certain size. So it's more polite to ask:

Is there a certain size you're looking for?