It is proper netiquette to send an email instead of texting messages longer than 75 characters.
Text messages are supposed to be short by definition. If it is more than two or three sentences it should probably be sent by email. Email is a format designed to deliver a message. Text messaging is a format designed to send a quick note.
Long Text Netiquette
Most Short Messaging Services (SMS), text messages, allow users to compose messages up to 140 characters long. This does not mean that it is good netiquette to use the entire amount of space.
The longer a message is the more room for error there is in understanding it. Many people receive text messages when they are on the go. Cell phones are the most common way to text. Reading messages on the go usually means that the recipient is doing two things at once.
If a recipient is multi-tasking, long messages can be confusing because the messages meaning may hinge on one word. Further, the composing long messages is troublesome because cell phone keypads are small. Small keypads make it easy to text the wrong words or characters.