Caging for nonhuman primates is a primary responsibility for caretakers. Your monkey will likely spend 75-90 percent of their life in some type of cage depending on the time you have to dedicate to one-on-one interaction with them.
You are limited only by the space you have to offer and the money you have to invest in your monkey’s living space. The larger the better but also consider quality products as your monkey will likely live 20-40 years or more depending on the species you plan to house.
Safety is a key concern as you don’t want your monkey injured or killed due to a faulty latch or inferior door system. Make sure your latching and lock system is tight as a loose door (feeding or entrance) could entice your monkey to attempt an escape that could cost his life.
Research what is available and select a product that is suitable for the species and size monkey that will be living in the enclosure. Consider what it is made of and how it is constructed and the coating, if any, that may provide a longer cage life. It is wiser to spend a little more initially than have the cage rust out in a few years and you are back to purchasing new living quarters. You may think you will always diaper your monkey and have no need of a product that will hold up to urine and the wear it causes but better to be prepared. Powder coated caging extends the life of the product. All powder coating is not the same. An industrial powder coat holds up the best.