Is it OK to chase payment by text or WhatsApp?
Yes, if that is how you normally talk to the client. Keep it short and polite, include the invoice number and amount, and follow up in writing by email if it goes past a first reminder.
How long can an SMS be?
One SMS segment holds 160 standard characters (70 if you use emoji or some special characters). Longer messages are split and may cost more. The counter shows segments as you type.
Will the message be sent automatically?
No. The button opens WhatsApp or your messaging app with the text filled in — you press send.