Who is that fatlarge man?

We don't really use "fat" to describe people any more in the US.

He is My's employee.

Is My the name of a person? If so, then this sentence is correct. If not, the correction would be this: "He is my employee."

They are air hostesseflight attendants.

This is what we call them in the US.

They are postmenal carriers.

They are policemen officers.

They are policewomen officers.

Who is that fat man?

This is not incorrect but it is considered impolite to say someone is "fat." Unfortunately we don't really have a way to describe a person's weight that is considered polite, although we might say that somebody is "overweight."

He is My'smy employee.

Since "my" already expresses possession there is no need to have an apostrophe here.

They are air hostesseflight attendants.

We don't call them "air hostesses" but instead "flight attendants."

They are policewomen.

We don't really use "policewomen" but refer to women in the police as "police officers."

Who is that fat man?

This is a correct sentence but "fat" can usually come off really rude. There's really no way in English to describe someone's weight without coming off rude. It's okay, though! I just wanted to let you know.

He is My's employee.

Unless "My" is a name, you would get rid of the "'s" and the capital letter because "'s" is usually used when you're using a name. You're right that it suggests that something is yours, but the "'s" is used when talking about someone else.

He is My'smy employee.

Unless there is a person named "My" who you are referring to.

They are air hostesseflight attendants.

He is My'smy employee.