The mattresses in the the two small bedrooms are custom made to measure and are 150 wide, the big one is 180 wide I think. But easy for you to measure to make sure. I thought we had three full sets for all the beds?
These sheets were bought in Chang Mai long ago but they are standard seize. You will need to check the quilts measurements as well. The bottom sheets should be "fitted" like the existing ones to make the changeovers faster and easier. I guess since you are on Koh Tao the only option for the moment is to buy from pen wholesale or similar... Yes it's very important the sheets are in good condition and good quality.