If you are unable to make it to any jewellers where they can let you know your size, your next best option is to measure the inside diameter of a current ring you own.
Measuring a current ring is a much better option than attempting to wrap paper around your finger and measure that.
To do so, all you need to do is place your ring on top of a ruler, and measure the widest internal part of the ring.
You can then compare your measurement to this table of sizes http://www.ringsizes.co
See diagram below for help.
If you don't own a ring that fits the desired finger, you can order a ring sizer from me here: