My experience in this area comes from spending years in Central America. Other Spanish-speaking regions sometimes have different words for the same thing, so YMMV.
"Pimienta" in Spanish typically refers to the dried fruits of the genus
Piper, e.g., peppercorns. "Pimienta negra" = black pepper.
In the same area, "pimenta" (note the important one-letter difference compared to above) colloquially refers to allspice (or the
Pimenta dioica tree from which allspice berries are obtained,) but can also refer to the genus
Pimenta which includes several other non-allspice species of the Myrtaceae family.
Can't say much about the sizes of berries, though in my experience with other fruits it's usually the case that larger fruits have a higher water content and therefore less flavor. If you find out which allspice is more flavorful, pls let us know.