Hi,
If you can't send long SMS, please go to Settings in AirDroid app, turn on "Fix SMS" and try again.
If the long message split into several parts on your side, sorry that this issue won't be fixed soon.
To fix this, you'll need to set AirDroid as the default SMS app. Since there's no SMS function in AirDroid app, you have to change the default SMS every time when you switch from sending SMS via web.aridroid.com to sending SMS directly via your device. Considering the inconvenience this may have caused, this will not be fixed soon.
Sorry again for the inconvenience this have caused