Now first I'd like to preference by saying I do not think that she has sunshine . Bartra said golden colored magic not sun magic.

As for her when she appeared in the picture of the prophecy she looked like a man. Well I'll say when she uses the full extent of her power her hair will turn white. As for the being big and musclely part... That could be armor. We've seen Gother have a giant suit of armor.

Now to what her power is. That's a tuff one, we'd need to keep the powers between the knights unique so I believe the power is solar manipulation rather than sunshine. Like she can bend and reffract light to make illusions or simply gather the light around her to form attack spells rather than absorb it to get big.

Now hopefully this makes sense