Weight is really gonna be the most accurate way to figure it out. The guideline I've seen most often is to feed a prey item (or equivalent multiple smaller items) equal to 10 - 15% the snakes body weight. Two weeks is also a long time to go between meals, and one mouse is NOT gonna be a satisfactory meal unless it's a pretty big one. You might find these charts helpful. General guideline for what to be feeding by weight and prey sizing equivalent.