This is not only possible, it is the default setting. If you are being asked to change partition size then the only explanation I can think of is that the installer is not happy with the partition alignment and needs to change it slightly to ensure correct installation. In all the years I have been using Ubiquity (and I last used it 3 days ago) I have never been asked to resize a partition just to select it as /.
Fujitsu Lifebook AH532. Intel i5 processor, 6Gb ram, Intel HD3000 graphics, Intel Audio/wifi. Realtek RTL8111/8168B Ethernet.Lubuntu 13.10,Ubuntu12.10 (Unity), Mint16 (Cinnamon), Manjaro (Xfce).