Hi Nithya,
I used 1.0mm cord for this project because that was the largest diameter elastic that would fit through the crystals on the strand. You'll have less wear and tear on the elastic if you match the cord to the bead hole size.
If you find the large holed beads (like Kashmiri beads) are floating around too much, try using something inside them. My favourite trick is to use hollow tubing inside. You might find this post useful for filling the gaps in large hole beads:
http://milllanestudio.blogspot.com.au/2011/08/7-ways-to-deal-with-large-holed-beads.htmlHope that helps!
Myléne