Mass Effect may be a good fit, actually! I'd say that the snag might be the length, but if you focus purely on the story the game might be only slightly above your threshold. If you decide to do the side quests then you're looking at a game maybe 4 times larger.

I'd say give it a go, but if you become snagged on a part have another, shorter story game on thr go to take a break from it. Splitting it up may be a decent way of going about it, if that becomes the case.