I think it’s 20 megabytes of characters, meaning that if you import a community track with 150,000 characters in its code vs. a track with 3 blocks, the community will take up a lot more space than the other. Generally, you should be fine with importing tracks for a long time. If you for some reason need more storage, the polytrack app has unlimited custom track storage because it uses your computer to store data.
The reason I guessed 20 megabytes is because that is the maximum size of a replay file before it will not be uploaded to the leaderboards.