when the heart is overstreched does the diameter increase or decrease and what does the contractibility do?

When the heart is overstretched by insufficient emptying during each contraction, overtime, the volume of the heart chambers increase and the overall size of the heart increases too. Individual cardiac muscle cells adapt by undergoing eccentric hypertrophy (grow larger). There is also an increase contractility (strength of contraction) by 1) Starling's Law (higher end diastolic volume leads to higher contractility of cardiac muscle fibers) and 2) hypertrophy. This helps the heart pump out more blood with each contraction.

There are two ways to answer your question.

!: If you are talking about an individual that does a lot of aerobic exercise such as running, the overall size of the heart increases along with the thickness of the chamber walls. The heart becomes a more powerful pump and has to beat less often to produce the same blood flow volume as in the average heart.

2: The second instance is a form of heart disease. In this case the heart will become slightly larger. This will be mostly due to the enlarged left ventricle. Due to a higher arterial resistance to blood flow the left ventricle must pump harder to overcome the added resistance. The chamber walls in this case are stretched without the same amount of thickening as in the first example. Eventually it leads to congestive heart failure. If properly treated and they stay healthy otherwise, an individual can live for decades with an enlarged left ventricle. The CHF usually ends up as the major cause of death in the very old.

increases, contractibility increases. see Frank-Starling law of the heart

i love starling's law!