They tend to run a little skimpier than regular undies. For me, most regular briefs I wear are in between medium and large. Large in MU is already kind of tight. For atomic - I'd guess one or two sizes above what you wear *should* do it.
In terms of durability... I've started doing chair wedgies in some of the briefs I own so far... the snap right back into place when the wedgie is over... so that's promising.