Twitter has recently taken its 140 character limit up a level, well, actually, a few.
Some people still haven't had the new Twitter update which allows you to send longer Tweets. This is because you're app is not up-to-date.
The good news is, most users have the latest version of the app. If you haven't, fear not as it is very easy to update, no matter what operating system you're using.
You may already have the update as on most Android phones, for example, have automatic updates turned on. This saves you time and having to update apps manually.
If automatic updates aren't enabled on your iPhone, go to Settings > App Store and turn the option on. For Google Play, the automatic update option is within the app in Settings.
You'll know if you have the latest version of Twitter as instead of seeing the 140 characters counting down every time to type or tap, you'll see a small blue and gray circle.