Access the Salesforce "Call Centers" menu within the Setup view, select the "PhoneIQ OpenCTI" name, and click "edit." On "Softphone Height" and "Softphone Width" input:

Width: 752

Height: 570    

These are the recommended parameters for a correct display of the UI. If you would like to resize the CTI, the best option is to change the browser zoom:

On the browser tab, click the control key (command on Mac) and use the mouse wheel or the '+' and '-' keys to change the zoom.