For ubuntu users..
One way to install gimp 2.8 is:
Firstly uninstall (remove) the previous version and then:
sudo add-apt-repository ppa:otto-kesselgulasch/gimp
sudo apt-get update && sudo apt-get install gimp
Single window view:
This can be turned on/off from the windows menu.
Brush size...I see 4 ways to do that:
1) Mouse over the size slider (in my pic the orange bit) and drag to the right or left.
2) Click on the arrows in the size selector up and down.
3) Click on the size number and arrow up or down.
4) Click on the size number (in my pic 20) and type in what you want.