max length of customer id is 255
max length of description is 350 (may change in future)
Copied from the doc,
You can safely assume object IDs we generate will never exceed 255 characters, but you should be able to handle IDs of up to that length. If for example you’re using MySQL, you should store IDs in a VARCHAR(255) COLLATE utf8_bin column (the COLLATE configuration ensures case-sensitivity in lookups).
The description field on customer endpoints has a maximum character length limit of 350 now