What size speakers does a 1993 Nissan Altima have?

http://www.mjmautoinnovations.com/Nissan that link could be helpful, will try to update how accurate it is in the next month when i replace my speakers.

Answer

rear speakers are 6x9 door speakers are 5x7 the rear dash must be removed to access rear speakers also make sure the magnet of the new 6x9s isn't to big or it will get caught on your trunk spring

Try to use the Nissan speaker adapters plugs so you don't need to cut wires and mess up the electric system, you can get those from eBay or amazon.