Right click "My Computer" -> Advanced system settings -> (under performance) settings -> Click "Advanced" tab -> (under virtual memory) Click "Change"
You can now manage the pagefile for any drive from here
If your drive is currently set to system managed, I would leave it alone. You can set it to a custom size but it will reduce performance. The system will only ever reserve space it needs which means that reducing that amount will force the operating system to move things around in the page file, which is slow.