Two things you need to do in order to enable resolution changes. shared folders and usb connections to peripherals.
Install extension pack
Install guest additions
How to install extension pack
- Check your version of Virtualbox. You can do this by opening it up and going to "help" and selecting "about virtualbox". You will get a splash screen that looks like this:
You will probably be the same version as mine. That is to say, 5.0.26
- go back to "help" and select "virtualbox website". This will cause your browser to open at the oracle site. In there select the "downloads" link on the left hand side. It will take you to a page that looks like this:
On the second paragraph, it mentions "extension packs". you need to click on the link to the extension pack that matches your version of Virtualbox. However, I have made a direct link to two of them below, to make it easier for you.
After selecting the link, it will ask you if you want to install it. choose yes and follow any instructions.
Installing the above now means USB access will work, as will shared folders. You still need to set them up. But installing the extension pack was a prerequisite to that.
How to install guest additions
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fire up your Win7 VM
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go to the vb menu at the top and choose "Devices/insert guest additions CD". See below:
You will get a windows dialog asking if you want to run Geust additions and you should choose to do so
When it is complete, reboot your VM and you should now find you can change resolution.
Also, as an aside, a quick way to go full-screen is to press Right-CTRL/f. If you want to go back to windowed mode, just press it again.