So, I use a Macbook and I installed VirtualBox and Ubuntu but the window appears very little (a tiny square in the middle of the screen)
I followed this question and I installed those extras they said there... But now how do I resize my window?
How do you resize the standard Ubuntu Desktop inside of Virtualbox?
Virtual Box should resize automatically after installing the guest additions. If this is not the case you might check if virtual box actually was installed correctly. It is not done with downloading and mounting the image. You will have to browse inside your hosted OS with the file browser to the mounted virtual Guest additions image and run it. If this was correctly done, check in the VB menu. There is an entryto adjust the window. With the guest additions installed it should find automatically the right resolution and size.