There is no official feature to limit characters in a text entry field. However, there is a super secret hack-ey way to limit characters without using javascript.
Insert Slide > Survey Question > Essay.
In Form View, you'll see "Maximum number of characters: 5000." Change that number to any number less than 5000. Image 1 shows maximum characters = 15.
In Slide View, there are two objects: a text box for the header (question) and a large data entry object text entry field. Delete the text box and resize the text entry field to the size and font desired. See Image 2.
The underlying structure is the same as inserting a text entry field using the steps: Insert > Input > Data Entry > Text Entry Field. In the end, it's a standard data entry object text entry field with a default "TextEntry" text variable. The advantage? Character limits.
Image 1: Essay question Form View show changing the character limit from 5000 to 15 and no feedback.
Image 2: Slide View shows the deleted header text box and reduced the Essay question text entry field. The trigger panel shows the default TextEntry variable trigger.
That's not currently possible using a built-in method in Storyline, but you can submit a feature request here.
You might also be interested in a similar discussion that took place in this older thread. You may see a familiar face there :)
Having said all that - I think there was some light mention of being able to do this with JavaScript. Though this isn't something I could help with, maybe others could jump in and provide some suggestions.
It's not yet a feature to restrict or limit the amount of characters, but you're welcome to share your thoughts in regards to as a future feature here.
Has this feature been added yet? I still can't find a way to restrict the number of characters input in an input text box. Will this feature ever be added?
There isn't currently a way to restrict the number of characters accepted in an input field. I hope others will share if they've configured a workaround.