Prepare your text extraction profile
Set up a QA rule for length control, to manage the size of your segments. This will allow you to setup a QA alert to inform the user he will need to adapt the text no to reach the limit you have defined for each segment.
You can find the options to define this in the QA tab of the text extraction profile, where a minimum and/or maximum value can be configured in different units (characters or percent).
The example below shows the configuration for .srt files:
You can even combine this rule with the Global Setting in the Translation Editor to prevent users from keep typing text, so that it is impossible to enter any additional character in the segment.