Typically there are three Battalions in a Regiment. A Marine Corps Grunt Battalion has on average around 750 Marines (may vary if the U.S. is in a time of war or not). So a Regiment would average around 2300 Marines.
Sizes vary with the structure and the structure varies depending on budget and the needs of the country. It could be up 10 regiments in a Division. The normal Table of Organization calls for three...
That can depends on what size of a unit you are talking about. A squad usually runs about 10 people. A platoon will have 3 to 5 squads. A company has two platoons. And a battalion normally has 4...