Change the size of a GroupBox

I am new in Qt and C ++, I have a code and I want to change it, when I open the file .ui i find a GroupBox , I tried to change its size in the interface but it does not work, I tried to change it in the code but I can't find it, is that normal that it is in the interface and not in the code? and how to change it if it does, I expect your help and thank you.