Limit Bank KFF value to 29 charecters in People's form

Hi,

Our requirement is to limit the IBAN field which is a KFF segment to 29 characters’.

Employee Form-- > Assignment--> Others-- > payment method

In Payment Methods we have Bank Details field which is a KFF. Here we want to restrict IBAN field value to 29 characters’.
The value should be 29 not less not more.

Please let me know the possible options of achieving it.
Any help is much appreciated.

Regards,
Shabana.