Just checked the app store and the file size of the app is only 40 MB which really isn't much. It says it uses raster and vector NOAA charts. Raster charts take up a lot of space, maybe 100-300 MB per region, but depending on where you go you may not need too many regions. I just use 4 or 5 regions for the entire east coast- Key West to Maine. Vector charts take up much less room, 20-50 MB per region.
So a rough guess based on east coast only, raster charts would be about a GB of memory. 300 MB should handle vector charts ok.
David