Dress size isn't something to measure yourself by as there isn't a standard. You could fit into a size 8 in one shop, but could be a size 10 in another or even a 12 elsewhere. Don't kid yourself it's a good thing to be going around saying you're a size 6/8.
Use BMI is an indicator of what your weight should be. There's a range for what weight you should be for your height and it's a much better indicator of what your weight should be than some random clothing size.