I've got a 22.5 inch head and a lot of Arda's wigs fit me without a problem when making the adjustable straps as tight as they can be.
When I need the wig smaller, it's a very simple process.
For their normal wigs (the ones not in the specialty section), put the wig on and pinch it at the back until it feels as right as you would like it. Take the wig off while keeping the back pinched and put pins on either side of the pinched area. Measure the area, distributing that measurement on both sides of the wig (it needs to be the sides because you'll be cutting the elastic), pinning it.
Get a seam ripper and rip out some of the wefts at the elastic where the pins are now (where you'll be cutting). Cut however much elastic you'll need to (you can go ahead and cut out the adjusting straps, you won't need those anymore). Get a needle and thread and see up the gap between the elastic, and then sew the wefts back on. You're done!
Sorry if I didn't describe this very well. ^^"
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