There are several options.

1.. It sounds like what you want to do is just this. It creates an array that can be resized/ edited in the inspector.

 public int[] foo;

2.. Or if you are trying to resize your array frequently in-game, you might want to consider using ArrayLists or GenericLists. Generic lists will show up in the inspector, but arraylists will not.

To access the size of an array use array.Length. To access the number of items in a generic list use list.Count.

ex.

if(foo.Length == 5) {
     print("foo has 5 elements");
}

To answer your second question you can get the size using the Length property, e.g.

int sizeOfArray = foo.Length;

Also, you can resize the built in arrays using static members of the Array class, e.g.

Array.Resize<int>( ref foo, foo.Length + 1 );  // increases array size by one

Though as mentioned, it is often easier to use List<> if you'll do that often. The nice thing about normal arrays is they're faster to access.