Hi  Jayasankar,

Based on the mentioned case, you will have to 

1) Add a custom class (with a space/blank after the default class) to the popup widget Style Classes property as shown below.

Ex: custom-popup-style

2) Add a class definition to the respective Screen Style Sheet section as mentioned below

.custom-popup-style {
    min-width: 70%;
}

Hope this helps you!

Kind regards,

Benjith Sam