Flat won't work, it needs to be round, and not wimpy elastic thread. It will look like white chord and by the yard so you'll need it to be cut, when I bought it was less than $1 a yard. When you pull on the elastic string/chord it doesn't give all that easy. Take the doll with you to make sure you don't get too big.
To roughly gage the amount, and this is what I did for a Dollzone, measure from one ankle up to where the leg string knot would be and down to the other and double. For the arm string from one wrist across the shoulders to the other wrist and double. This will give you extra string and make the doll very loose but that's easier to fix than too short. Gently pull on the knotted end, gently tug on the arms and legs to spread the tension out, make a new knot further down the strings and cut off the spare end.