The one paragraph you have there does not explain your idea clearly to me. And why do I need 5GB of storage for a discussion?
Since we’re an ad-free platform, there needs to be some sort of way to make money from it. I thought, in an age where most people are simply fed up with the world of ads, why not go back to square one, and start as a subscription-based model? And, not to mention, I’d say most people won’t have the need to upgrade to a paid plan anytime soon after they start posting content, anyway. 5 GB goes a long way!
I think it is a bit early for you to worry about that problem, or at least show your worries to your users. You need to show the value to your user before showing the cost.
Please note that this platform yet has no data populated. We'd appreciate the early population of interesting discussions, to make the site a living, breathing web app. Feedback is appreciated!