Brief Boxers

rule of thumb for ANYTHING, Tight if ur ripped, Lose if ur not. for boxer briefs, just make sure the tight ones have ample space for ur junk.

I'd recommend going with a trunk cut over the boxer brief - it's a bit more square in it's look vs the slightly longer leg of the b.brief.  I've been using these for working out / HIIT / Lifting and they're solid.:

Adidas 360 Trunk

Also, I think loose for out of shape and more fitting if you're cut isn't bad advice.  Also, if you're wearing slim / lightweight pants like dress trousers the last thing you want is underwear that's bunching / riding up.

For the love of God, don't buy your briefs to suit a woman. Buy them for your comfort and support. If those two things overlap then great! If you're heavy, don't get boxers that will make you look heavier. Getting the feedback from a snugger fight might even encourage you to drop a few.

I love Adidas Climalite boxer briers. They come in at under $8/pair from JC Penny online and have lasted through marathon seasons and lots of gym work. I wear em under business casual clothes, with jeans, basically everywhere, and they are invisible.