When should you consider getting a mole checked at? is there a size that is worse than others?

If it's changing.: In general, we worry about moles that are changing in some way. Itching, bleeding, sudden color change, and rapid growth are all red flags. As for size, 1cm (the width of a pencil eraser) is usually quoted as a minimum, but a smaller one that displays any of the above should also be checked. If you are worried for any reason, have it checked, preferably by a dermatologist.