Verify the validity of custom infopath data

In infopath, you can set the maximum length of a text box in the attributes of the text box,
Attribute:

Verification cannot be empty.

Property-Display

Here we can see that infopath Forms Services does not support those settings.
You can right-click the control and choose "Data Validity" to add data verification conditions.
If you need complex verification conditions, you need to writeCode:
Right-click the control and choose programming> validating event.

Then you can write your own code in the opened code.

Would you like to give it a try?