I am planning to design some patterned wall paper for the walls of my office and I was wondering if there are recommended dimensions for the artboard when designing it via illustrator.
Your design in Illustrator should be in all vector, so the size doesn't really matter. I personally find it very helpful to put an image of the object I design for at the bottom of my layers.
So get an image of your wall(s), lock it and design above it. For printing/projecting it on the wall, you can resize your (vector)work in any size.
The other answers are correct, it doesn’t matter that much as long as it is proportionally identical. Working on 1:1 scale, wall sized may give you performance issues though. You should be aware though that Illy only has an accuracy of 3DP. This can cause some problems with scaling when things need to be super accurate. Some effects and things (especially Warp and Mesh) do a lot better at larger sizes.