What can be done to dramatically reduce the size of the top central incisors? (Photo)

There appears like great progress on the orthodontics! I would say congratulations on picking a good orthodontist, believe me it is difficult to work with an excellent one.Option: Leave the centrals alone. Increase the size of the lateral incisors instead, with crown lengthening and/or veneers. Option: Intrude (move apically) the lateral incisors. If you do that, there will be a step down, or shortening of the edge compared to its existing position. There is nothing wrong with that, it is just a variation in the position of the laterals (my teeth are setup like that).Option: DO NOTHING. Its looking pretty nice  AS IS.As with all information, consult a professional (Specialist) before proceeding with your dental care.

Porcelain veneers should solve your problem. You can adjust the proportion of the 2 front teeth by adding a couple veneers on the teeth behind them to have an even proportion of all of them. Its good that your teeth are aligned now at least so that your treatment in veneer design can be more conservative and you will have better results. 

its my advice to wait for sometime after braces removal. because some changes will occur in soft tissue [gums].

the problem  is not only the size of length, is also the width, as you can see they are more wider than the lateral ones, this makes them look bigger, this would not only be solved with 2 veneers, i think that it needs more a smile design, and determine  how many veneers need to be done, to make a smile even, with veneers you can change shape, size and even color.

Although an orthodontist looks at the teeth and esthetics, their main job is to straighten teeth. I would recommend you find a good general dentist who does cosmetic dentistry and talk to them about your concerns and desires. There is a " golden  proportion" to front teeth that is the most pleasing to the eye. the dentist can measure your teeth and  let you know where you stand .I fthe centrals are too wide ,they can be narrowed by disking or removing some enamel from in between them on both sides. From the picture you posted, they don't seem that much too big if any. Please see the dentist and make a decision on what you want so that the orthodontist can be communicated with so you can get what you want.