Three general guidelines
1. Keep 'em short. Think less European man-thong and more fitted trunk that hits two or three inches above the knee. It'll make you feel confident, appear trimmer, and not look like a prepubescent tween who's sporting a body-engulfing suit with illogical neon flames streaming down one side.
Pro-tip: If you feel uncomfortable sporting shorter trunks, try them with vertical stripes. They'll make even the most truncated suits appear longer.
2. Avoid elastic. Since most of us are not models with picturesque frames, look for a suit with a flat front that you can tie and control how much it’ll cinch your glorious gut (regardless of how small or large it is). If you've been working on those abdominal muscles all winter, you can also opt for a suit that buttons. Otherwise, stick with the waist-forgiving drawstring.
3. Know your inseam. Swim trunks generally come in one of four inseam lengths (in inches): 5, 7, 9, and 11. In most cases, all four of these will hit above your knee (unless you're very uniquely shaped). To figure out which inseam is best for you, take into consideration your height, stature, comfort level, and most importantly, the amount of thigh freedom you need to high-kick like Jonathan Brandis in Sidekicks.