different text desktop / smartphone

Hello, How can i change the article (amount of text) to be shown on smartphone? 
I have a large website with a lot of text in the articles. I want to have a small version of the text visible on a smartphone.

Most likely the only way it to write a plugin that will cut article intro text after certain amount of characters. Try search on JED, maybe such a plugin exist. Then in MJ Advanced settings you can set this plugin to work in mobile mode only.