The camera doesn't measure it in terms of raw storage, it measures in terms of how far back in time footage can (typically) be recorded, as this is usually the real concern that customers have. If you navigate to the camera of interest, in Dashboard, then choose Settings > Quality and retention - at the bottom of the page it tells you the expected retention time for that camera, given it's configuration, field of view and past history (because, for example, how much motion you get will vary between cameras with the same configuration, if you enable motion-based retention).
Note too that you can examine how that retention period might change by just trying alternative settings (without saving). Again the camera will look at past history to extrapolate forward with the proposed changes. In my case, for example, my MV72 (motion retention enabled) reports 44 days retention. If I click disabled (camera now retaining all footage) that drops to 15 days. If, instead, I switch to Enhanced video quality (15 fps, instead of 24 fps) it jumps to 90+ days (we don't estimate anything beyond 90 days) Read more here:
https://documentation.meraki.com/MV/Initial_Configuration/Video_Retention
Bear in mind that, assuming a camera hasn't just gone in, it's storage will be pretty much always full - it will be overwriting that footage which is least valuable, given it's configuration (typically the oldest footage)
Hope this helps