My conversation screen changed size. How do I fix it?

When I click and open into a conversation screen with someone my screen is taller than it used to be. It is so tall that where I am typing is no longer visible to me, the screen goes below my tool bar. How do I make the conversation screen the size that I want it to be? I have tried everything I can think of.

I have Windows XL. Be very detailed with your answers. I am not very good at computers, I'm still learning.My conversation screen changed size. How do I fix it?
ummm try going on the lines to make it small you should see arrows and then pull it to size of your choice