How boxer briefs should actually fit
Most men wear underwear a size too tight and call it support.
Start with the leg. A boxer brief leg should end mid-thigh and stay there when you sit, climb stairs or get out of a car. If it rides, the cause is almost always a leg opening too small for your thigh, not a waist too big.
Next, the waistband. Pull it out 3cm and let go. It should return flat in one movement. A band that folds is under-built; a band that digs is a size too small.
Finally the pouch. Seams should sit beside, never across. If you can feel a seam, the pattern is wrong for your body and no size change will fix it.
Relevant to this guide
The Boxer Brief