Then you have to pick those characters that you consider important and cut off the rest. The first 30 was just an example.
In your example you mark Size: 19.8 GB, 92.04% used as important, but the check isn’t WARN because of the 92.04% but rather because only 1.58 GB are left.
How would checkmk know that? I’m afraid you have to modify your SMS sending tool to only consider what you think is relevant.