As far as I know a text message is fixed in length but you can use something like whatsapp which sends messages using your data connection. The app is free and cross platform.
Contacts need to have it installed also.
You are right to say that WhatsApp is a very good app for texting and also to increase the size of the text. WhatsApp can be used and if you have any problem with WhatsApp, you can use this FMWhatsApp Apk modified version of WhatsApp. To increase the size of SMS