I am a low supply BM mummy. So when I return back to work, I assume I may only expressed into a 160ml bottle. This is based on I can only express maybe 2 times during my office hour. If that the case , will a cooler bag be necessary? Or is there any brand of cooler bag which is smaller ?

Does your office have a fridge you can use? That way, you don't need a cooler bag. In any case, keeping the expressed milk chilled (no matter what amount) is important. Ebm can last 6-8 hours in room temperature if the temperature is on the cooler side but you can never be too sure with Singapore's climate even if it is air conditioned. The Medela cooler set is perfect for working moms. It's small enough to carry around and keeps breastmilk cool for the amount of time you are at work. Retails for about 60SGD :)