Well you don't actually adjust your buffers in Cubase, they are set within the Control Panel of your audio hardware. To open that from Cubase:
Goto Devices Menu> Device Setup.
This will open the Device Setup Dialog. The left hand column are all the devices available to Cubase. Choose "VST Multitrack". The hardware you are using will be list in the ASIO Driver box, and below that will be a button labeled "Control Panel". But unfortunately, with 99.9% of all hardware out there, once you change the buffers you will need to restart the program for the changes to take affect.
Quicker method: Open your control panel for your hardware (Without Cubase running), and if it allows you, create performance profiles (Most do).
Example: Create a profile for Normal use (As your probably using now), and create a Profile with Max Buffers (I named mine Overload...as that what was happening with normal buffers to make me open up the control panel), and maybe do 1 with minimal buffers, and call it Live or something. I usually work in "Normal", and when I have to switch to "Overload", I know it is time to be thinking about bouncing down some VSTi's.
