Size: 5 or 6 mm. Only because it's the size my dad had. I think 4 looks really thin and 8 looks really thick, but it's all personal preference, definitely try a few on. Maybe buy some really cheap ones from Amazon and to see what width you like.
Metal: Titanium. I actually wore a tungsten band when I got engaged for a while ($30 on amazon), but it was pretty heavy, and the edges were sharp and really irritated my pinky finger (slowly wore the skin away). When I got the titanium band ($100) it was so much lighter, definitely worth it. You think it's silly and you won't notice, but you do. The fear about titanium being unbreakable/uncuttable is misinformed, both hospitals and jewelers have tools to remove titanium rings. Titanium rings are not made of the same intensely hard alloys as rockets. The only downside is that you can't resize them. Also titanium is way cooler (in my mind) than gold or platinum (though palladium would be cool, but it's also pretty expensive).
Also I don't power lift, but I went through a phase of working out a bunch with free weights, and it was fine with a ring, it got scratched a little, but I shudder to think of how much more scratched it would be if it were gold or silver.
In conclusions: Go with Titanium. Don't buy it from a jeweler.