Increasing the window length upon click of subgrid + button

Hello team,

I am using one of the system entity and having a custom subgrid to another system entity.

Upon click of + button in he subgrid it is opening a new window on top of he maim window. The new window size is half of the main window,  now I want to increase the size od the new window. There is a maximize option but that doesn't really help because every time user have to maximize, instead I want to increase the size of new window.

I am not sure how to do this.Any help or guidance is highly appreciated.