Measure head size to determine target circumference. Depending upon your personal preference, you might want to upsize or downsize a bit. For length, measure from your forehead to an inch or two below base of neck depending on your preferences.
If there is need to resize the circumference of a smoke ring pattern, it will usually not be major amounts such as might be needed for other parts of our bodies! One of these strategies will likely suffice —
1. If the pattern stitch multiple used in the pattern is a relatively small number of stitches (e.g. a multiple of the stitch pattern in the planned gauge is approximately equivalent to the measurement you need to adjust for), just add or subtract the corresponding number of stitches for that multiple.
2. If the pattern stitch is quite large, it might be adequate to adjust up or down by a needle size (or ½ needle size as available in some of the metric sizes). The drape of the fabric desired for a smoke ring is usually within some tolerance of a couple of needle sizes, and exact fit is not quite as important as if we were doing a sweater or socks.
Important! Remember that finished sizes for lace are taken after the lace is blocked. So be sure to take this into account in your project planning and sampling.