Boards are generally measured by width, and full size boards range from 7.5 - 9.0” and up, and are usually around 31.5-32.5” in length.
“Mini” Boards are for the youngest skaters around ages 6 and under, and are around 28-30” long and 7.3-8.0 wide. A wider board provides more stability and real estate to land on, but adds extra weight and bulk. A more narrow board is easier to flip and maneuver but has less room to land. As a point of reference the most common board sizes for average adult males 5’7” - 5’11” are 8.1 and 8.25”, and for adult females 5’3 - 5’7” sizes 7.75 thru 8.0 are most common.
Transition skaters that ride a lot of ramps and bowls generally lean on the wider side of things, and more technical skaters tend to prefer smaller size boards. It’s all personal preference, and the more you skate the more you’ll learn what’s best for you.