depends on what your looking to achive.. if your looking at installing v-groove style wainscotting or bead board you can get it in 32" tall or 8' which you cut to the length you need. theres also 4x8 sheets that you can get
the general rule of thumb is that the top of it should be 1/3 the total height of the wall.. in much older homes it can be 2/3 but works best if the ceiling is 9' high or greater if not it makes the space feel much smaller
Was recently in an older home with high ceilings. It had very tall wainscoating (wasn't bead boad) with a small shelf around the top. Looked pretty smashing to me. I know is was over 6 feet as I couldn't see the top of the small shelf and the ceilings were at least 10 feet.