Extra Small bindings are built for youth and very small adult boots, typically fitting US Men's 4-6 or Women's 5-7. At a Women's 7, you're right at the upper edge of the XS range, which means you could potentially fit into a Small as well depending on the brand.
The real concern with sizing is making sure your boot is secure. If the binding is too large, your boot can shift inside it, hurting your control and responsiveness. If it's too small, the boot overhangs the baseplate, which can cause your toe or heel to drag in the snow when you carve.
Since you're on the border, definitely check the specific brand's size chart—ranges vary between manufacturers. Also consider your boot's bulk: a thicker-soled or heavily padded boot may fit better in a Small, while a sleeker, lower-profile boot should work fine in an XS.