1. Most wigs will fit all dolls above size 140cm. The smaller dolls (smaller heads) may need a smaller wig or a fit band that tightens the wig. You’d have to check the wig brand for details.
2. Make sure to get a wig cap and usually go for wigs for smaller size heads. If the website you got the doll from sells wigs, you can order from there.
I have normal sized wigs that usually seemed to not stay on as well as the smaller ones.
3. The fit band can be a pair of hooks that attached to each other on the underside of the elastic band which will make a tighter fit(not all wigs have them). Some hooks are used to attach the wig to hair clips or loops along the underside.