Size, actually battery volume, and use time (in units of mAh...milli-amp-hours) are roughly directly proportional. So you won't find a 3.7V battery of the same size as your kr8 batt that lasts longer, given the same carto resistance and voltage under load (what the coil actually feels). Technically the same size 510 batt will last longer, since the coil is actually feeling 3.2V rather than the 3.7V a kr8 coil feels, but that is a detail.
I would recommend any of the 510 fat-batts, such as the eGo, Riva, Tornado or KGO (there are some others now too). Your kr8 might be as much as 280 mAh, but the fat batts start at about 650 mAh, and can go up to 1300 mAh, which are longer than the 650 mAh fat batts. My KGO 1100 mAh with a carto is about the same length as a kr8 XL batt with a carto. These will generally come with 510
threading, so you would get 510 cartos (I recommend Boges) or an adapter to use your current kr8 cartos. I don't think the kr8 batts go past the XL or "mega" size, which is about 280 mAh.