First off, I want to congratulate you on having the forethought to add exterior sconces flanking the French doors. This will help keep them from becoming black mirrors at night and add a little architectural jewelry to the facade. As far as a junction box goes I would use a round 3-inch box. This can be a noncorrosive metal box or plastic. It would be really good if you could pick a fixture ahead of time before installing the boxes because that will help greatly in determining what the correct mounting height should be.
A good rule of thumb is that the center of the light source should be 6 feet above the patio or deck (whatever the French doors are leading out onto). Some exterior fixtures do not have the electrical feeds exactly in the center of the backplate; sometimes they are closer to the top. Also, some fixtures hang down from an arm, or are raised up on an arm. These configurations affect the correct mounting height.
For example, you’ll see the ELK Lighting Freeport fixture (left) has the electrical feed toward the top of the backplate. It’s not centered on the backplate. In contrast, the Quoizel Oasis lantern (center) hangs lower than the junction box. So in this case, the junction box has to be mounted higher than 6 feet so that the light source ends up at the 6-foot height. The Sea Gull Wynfield lantern illustrates a light source situated above the backplate. For these styles, the junction box would need to be installed lower in order for the light source to situate at 6 feet.
My advice: Just bite the bullet and find the fixtures that you’re going to use. You need to buy the horse before you hitch up the cart. It will also help knowing the size of the backplate so that you can cut the right size opening in the siding.