What level are you? Do you still have more things to unlock? You have to keep that in mind that as you unlock new stuff it will take up even more space.
I don't think 500 is big enough, but it depends on how you play. I like to keep a lot of products on hand. For awhile I did stop upgrading my silo and would sell all of my silo materials to save barn space, but I kept my barn materials.
Make sure you don't have things wasting space too. A lot of times I see my barn is full and then notice I have a ridiculous amount of milk or animal feed or anything else that I can sell and easily make more of when I need it.
gc: kewb lvl 52