The extra room, unless it is a massive amount (I am picturing my 6 year son in my 20 degree bag - 45 inch tall kid in a 78 inch bag = way too much extra space to heat up!) it won't cause an issue with being cold, etc.
The opposite problem can cause issues -- if the bag is too short, and you are basically bottoming out the footbox (feet pressed up against it tightly) I have found that issue is worse, and creates cold spots due to the compressed insulation not creating air pockets.
Regarding a quilt, test drive the quilt via the offer of Colonel00; I recently upgraded to an EE 0° bag, and it is warm, light and comfortable. I am a side sleeper, and toss and turn a bit at night. No issues in the quilt.
Also, your sleeping pad plays heavily into the equation of staying warm in variable conditions -- based on the nighttime temps, be sure to have a good pad with a good R value to boost the overall sleep system.