Size is a question of personal preference however it has a huge impact on the price.
Akoya pearls
2-10mm. Most of the small-size production of 2-7mm now comes from China but better quality goods and bigger sizes of 7-10mm are still cultivated in Japan. Sizes of 10mm and above are extremely rare. General size tolerance: 0.5mm (e.g. 7-7.5mm or 7.5-8mm).
Freshwater pearls
2-7mm in cultured but seedless pearls but much bigger, even above 10mm for nucleated freshwater pearls. General size tolerance: 1.0mm (e.g. 3-4mm or 6-7mm).
Black and White South Sea pearls
9-18mm (in rare occasions up to 20mm). Size tolerance: 0.5mm in sizes up to 13mm and 1.0mm in sizes bigger than 13mm (e.g. 10-10.5 or 10.5-11 and 13-14 or 14-15) Single loose South Sea pearls are often measured to 1/10th of a millimetre such as 13.2mm or 9.8mm.
Keshi pearls
2-4mm in Akoya Keshi, 4-12mm (in rare cases bigger) for South Sea Keshis. Size tolerance: does not apply as all pearls are different in size and shape.
Conclusion
The larger the pearl is, the more unique, the rarer and thereby the more expensive it is.