@windstruck, you've done gem storage of energy before, including recharging them, so you can draw on that easily enough for this project.

Regarding expanding the field, there are two ways you could go about it.

First, you could define in the enchantment three different sizes (normal, personal and area) and simply set up triggers that move it from one size to another, at which point it would also draw more energy as needed from storage.

The other would be to not define the size at all, but have it scale based on the available energy. You could then add additional enchantments tot he storage medium to set a trigger to increase or decrease energy provided. This second option ahs the advantage that a spellcaster could feed in energy of their own to increase the field's size, and even fine-tune it a little based on requirements.