Yep, Jotunbrud from Races of Faerun. Essentially, you're a biggun. Descended from frost giants.
In a nutshell, you can count yourself as a large creature for the purpose of opposed checks (such as grappling, tripping, overruns, etc) if it is advantageous to you. Likewise, you can be treated as large for the purposes of abilities that are limited by a creature's size, such as swallow whole.
Unlike Powerful Build, it does not give one the ability to use larger weapons, deal unarmed damage as a large creature or anything of that nature.
It's also important to note the significance of the inclusion of can and if it is advantageous to you in the mechanics of this feat. If it's potentially better for you to be treated as a medium creature, then you're medium if you so wish it.