All other factors being equal, a larger pearl, is almost always command higher value. Pearls, are measured in millimeters, by their smallest width, shown in the picture below. Typically sizes are generalized, meaning that an 11mm pearl, can be anywhere from 11.00-11.99mm.
Pearl Size Ranges:
Different pearl types have different size ranges. For example, a 9mm pearl is considered to be exceptionally large by Akoya standards, but by South Sea standards it considered to be small. Below are the general size ranges of the different types of pearls.
More often than not, a strand of pearls will gradually vary in size. Typically, this means that the center pearl is largest (and at the forefront of attention) and the smallest pearls are in the back. The amount of graduation can be classified as slight, (one 1mm) medium (within 3mm) or heavy graduation (over 5mm) or no graduation at all, indicating that the pearls are all the same size. (within 1mm)
Necklace Strands typically vary in length, depending on the wearers personal style and preference.