SMS Helper is another amazing FREE application found on cydia repository that allows you to count the number of characters as you type a text messages in the default sms application provided with iphone. After installation it will require your springboard to restart so that the changes come into effect, it adds a counter into the stock Messages application.
The character count is displayed on the Send button in the lower right corner as you type the message. The first is that most mobile device carriers allow a text message of 160 characters it will display a counter upto 160 characters
Here is how you can install this application from cydia repository, launch cydia on your iphone, click the search tab at the bottom and type SMS Helper
After you locate this application, it will show the following screen
Note: As declared in the warning message it will not work with iphone with OS 2.0 or lesser than 3.0 you must have 3.0 or newer version of firmware to run this application.
Now as you can see in the image below as I type it shows me the number of characters I have used out of 160
Found In: Cydia Cost: Free