By default, translation length is unlimited. To limit the maximum character length of your translations for a specific key, find the desired key in the project editor and click on its name:
You will be presented with a key editor. Proceed to the Advanced tab and tweak the Character limit setting and then click Save:
Alternatively, you can choose one or more keys right in the project editor and choose Character limit: set/unset action from the bulk actions dropdown:
Please note that if you put a character limit on a key and then import updated translations for this key in, say, JSON format, or as an Offline XLIFF, the translation will still be updated even if the entry is longer than the character limit.
However, if a user clicks on the imported translation longer than the character limit, they won't be able to save the entry until the value is within the limits.