I had a Hypermaniac Ishta, and I found that small-medium-sized wigs fit best. Cherishdoll SD-sized wigs fit a bit on the tight side, as do Iplehouse, whereas Luts and Leekeworld were HUGE.
If you want to use a bigger wig with a small-headed doll, not only would I suggest the rubber-band wristlets (which it looks as though you are using), but also Volks Pitatto Wig Stopper (http://store.volksusa.com/matlst0004.html
). My girlfriend and I keep this on all of our dolls because it can really
help wig-slipping issues.