Could you please go into more detail on what you are trying to do? At a high level yes an App can easily send a SMS though the question comes into how you want to manage the phone number to send the message to. That is why most are creating virtual Twilio devices and have the app send a notification to that device. You could have an app input with phone numbers and send them to that list too. It will all be done via Restful Post message. Regarding length of message, that is defined on Twilio API documentation:
https://www.twilio.com/docs/sms/api/message-resource#create-a-message-resource
If the body of your message is more than 160 GSM-7 characters (or 70 UCS-2characters), Twilio will send the message as a segmented SMS and charge your account accordingly.
You can view code samples of the original Twilio driver I created before this was supported out of the box by HE:
This is done via driver but again you can take the web services calls and put them into an app.
