i wish you read this before asking ^^ :
http://www.scirra.com/forum/how-do-ifrequently-asked-questions_topic45416.html
there is a topic about your problem...
anyway the answer you can use lerp(the size,the next size,dt*(speed))
or you can use global variable that screen always follow that global and when you click button it will make gruop active , the gruop has everytick add one to that global.