3-15⁄16" (100mm) is the maximum height of an official FLL table's walls.What will be at your competition may vary depending on the competition, but all the walls will be the same height at that competition.
For those that are unaware, the rationale for the large tolerance in the wall height enables FLL to be an international program. Standard wood sizes vary from country to country. For example, in Europe, you cannot easily get a 2x4 like you can in the USA.
Technically, in the USA, there are two common dimensional lumber sizes that can be used for the walls of a FLL table (between 64mm and 100mm tall): a 2x3 and a 2x4. The actual sizes of these two are 1-1/2" x 2-1/2" (38mm x 64mm) and
1-1/2" x 3-1/2" (38mm x 89mm).
You best bet is ensure your robot can perform with a wall which is either short or tall. You can also ask the tournament coordinator what wall sizes they intent to use.