If you turn 1/3 and 1/6 into a decimal, it will be easier to tell.
Just take the numerator (the top number) and divide it by the denominator (the bottom number).
1/3 should be the bigger one.
Another way you can tell it's bigger is by drawing a tape measure.
(Draw a rectangle and cut it into 3 pieces)
(Draw another rectangle and cut it into 6)
You see 1/3 is larger than 1/6.