how can i increase the size of a pop up window in Opera?

when using latest Firefox or IE11 , its bigger and displays all the information i want on pop up window opening
in Opera it opens about 70% of the other browsers and i cannot see everything until i stretch the window size

so that takes two actions to get to where i want to be in Opera and only one in the others
i have contacted the website it originates from and they have amended it to make it bigger when opening but its no different in opera

anyone any thoughts on that issue and any tips to resolve it?

thanks in advance