What is the best sized crochet thread and/or yarn to make a bikini swimsuit?

your best bet would be to use cascade fixation or paton's katrina yarn becouse both of these have some elastic in them so they will fit better that most yarns. if you dont use a yarn like those i would suggest a sport or baby wieght so the stitches would be smaller so it wouldnt be see thru. and it should be soft because its gonna be so close to your skin. hope this helps