Sterling Silver Italian Ball Bead Chain 1mm 1.2mm 1.5mm 1.8mm 2.4mm 925 Italy New Dog Tag Necklace

Are lengths of necklaces measured open or closed

can this be gotten with a lobster clasp?

Is this chain hypoallergenic?

I am looking for a ball chain that would hold multiple charms. In order not to stress the chain which size would be best?