If your snowboard boots are too big, you will have a difficult time controlling your board. Too much space in the toe box can make it difficult to keep your toes from slipping around inside the boot, which can lead to loss of control.
If your toes are touching the end of the boot or if there is a lot of space in the toe box, the boots are probably too big. You also want to make sure that the boot is snug around your foot and ankle, with no gaps.