Something is said to sink in a liquid if it falls to the bottom. The olives in the martini in the picture have sunk to the bottom of the glass. Things sink because they have a higher density than the liquid they are in. The opposite of sink is float.
You sit down when you put most of your weight on your bottom, on a surface like a chair or sofa. Sitting down is usually more comfortable than standing up.
Size is a measure of how big something is. You can measure size in defined units like inches, or you can use descriptive words like big, small, tiny or huge.