Two retainers with 175 slots each, plus 20 market items each (If you are really desperate put things up for sale at 999,999,999 gil), 100 inventory slots, the armorer chest in the inn, and your equipment inventory. This much inventory space and people still complain about not enough space, Personally I've never even come close to running out of space, just have to get rid of things / sort it on my retainer, ect.