I think a stash can be too big if you can't manage it. To me, a fabric stash is one that is relatively organized, and I can see it or know where it is. So in my opinion, fabric in boxes stored in another room is not a stash unless the box is labeled & you know the exact contents. Not everyone has a huge space or dedicated storage to stash.
But to me, when it gets to the point where, out of sight is out of mind, you buy more of the same because you can't be bothered to look for it (emergencies notwithstanding) and the joy of having the fabric is gone after it's purchased and stuffed into a box never to be seen or heard from again... well, to me that is hoarding.
For myself, I'm probably on the extreme end of organized; every piece of fabric over 1/2 yard has an inventory tag, the price I paid when I bought it, the date of purchase, and the yardage. Periodically I will go through and donate or sell the fabrics that no longer inspire me so I can keep my stash "fresh".
I've also been the other way, where I had so much that I couldn't find what I wanted and it was so incredibly frustrating and crushed my creativity to not be able to put my hands on something that I knew I had.
Most people I would say know their own limits... and your stash is too big when you think it's too big; when the joy of seeing the stash you already own is a distant second to the new fabrics you want to buy.