Yes, I understand what you say when creating a new user attribute, integer or Floating-point numerical type. You can define de minimum and maximum value of this attribute can have (see image1)
But in the case that I say, I think something different works.
If we look at the "Layout user attributes", from the attibutemanager.exe or inside the "LocalLayoutAttributeDefinition.xml" file, we see that the maximum value is 63 characters, and if we try to put a higher value this message appears: "You can only enter up to 63 characters in this field..." (see image2)
In fact, if we modify that maximum value by editing the xml file for example to 10 characters, we see that the message is in accordance with this modification: "You can only enter up to 10 characters in this field..." (see image3)
Anyway, put the number of characters that you put, if you create a Legend with these attibutes, the result is always truncated to 63 characters.
that's why I think something different works.