Doesn't a cancerous mole have to grow or change in size?

Yes. They keep growing. They need to come out. See dermatologist.
ABCDEs of Melanoma. The abcdes of melanoma are:a=asymmetry(one half unlike the other half), b=border (irregular, scalloped, poorly defined border), c=color (varied from one are to another), d=diameter (while melanomas are usually greater than 6mm they can be smaller when diagnosed), e=evolving(change in size, shape, color).Hope this helped :].