It depends on the length of your runs - determine how far the furthest fixture away from your transformer is a base it off this. As Mesodude said, the most common is 12-gauge which is generally rated for uses up to 150'
10-gauge would be ideal up to approximately 250' - working with LED is quite flexible given the wide input voltage ranges so the exactness may vary project to project based on load and other factors. A voltmeter is certainly a handy tool when installing a system.
