Here’s the best pictures I could get. I didn’t have a ruler so I couldn’t measure unfortunately. The pearls feel a bit heavy. Any hints of what these could be that will be swell
Do you have a UV lamp? If so, that is a quick and easy test for pearls. Real akoya/freshwater/white South Seas and other "white pearls" will "glow" (fluoresce) with a blue-green color, whereas imitations will not.
Use this video to check the effect:
I agree with TriciaS it's a very nice necklace. And being 100 years old is definitively something special. The 1920's, cultured (and faux ones too!) pearls were showing up, the Flappers were all over the place with long ropes!