I would think that a depth of one foot, starting from ground level would look in good proportions against the wall and would provide more than adequate depth for most annuals and perennial
flowers. At this depth, I don't think there would be any need to disturb the native soil, just backfill with a well draining, high quality planting mix. For
flower boxes and container gardens, I like a mix of equal parts bagged topsoil, mushroom compost and soil conditioner.
Width, I'd go at least a foot also, more depending on your personal preference.
Sounds like a beautiful project. Good luck.
