For men, they should fit snug around the waist and just barely relaxed around the hips and on the legs. The waist should sit where you normally wear dress pants or slacks (just above the hips).
The length for breeches is usually short. They should come to the top of your ankle bone, or slightly higher. Wear tall boots or paddocks with half chaps over them.
When you buy breeches, it's by waist size, just like jeans for men. However, they can run big or small, depending on the brand and cut.
If you want something that fits more like pants and you don't have to wear tall boots with them, then check out Polo Pants. My DH has a pair of these and really likes them. There are no patches or full seat, so they're not as grippy to ride in, but they're very comfy and don't have the bulky seems like jeans have. TuffRider Jaipur Polo Jeans-Mns and Polo Jeans*|*EQUESTRIAN COLLECTIONS.COM PRO POLO JEANS and Polo Jeans*|*EQUESTRIAN COLLECTIONS.COM EQTACK.COM - IRIDEON RIDING WEAR AND PASSIONPOLO JEANS - Boot Cut Breeches, Stretch Pants, Full Seat Breeches, Knee Patch Breeches and Passionpolo Jeans.
These polo pants have a self-patch knee patch: Polo Jeans [10052-14] - $53.95 : BreechesUSA