Finding a bivy that's just your size makes all the difference on colder nights.
Get the right size. A sleeper that's too long won't keep your feet warm because there's too much air movement at the bottom of the bag. A bag that's too short won't keep your feet warm either because your toes are pressing against the bottom, flattening the insulation.
Keep in mind that a snug-fitting mummy is easier to warm up than a roomy one, especially if you sleep cold or do a lot of winter camping.
Go with the longer bivy if you're on the fence between two different lengths. A little extra room in the bottom of the bag allows you to store items that you want to keep from freezing, like cameras or water bottles.