The minimum opening to a shower enclosure should be 24" in width. Wider is easier to enter and exit. Your term, "knee wall" is a bit confusing...we use the term to mean a short wall supporting a floor system, like in an elevated crawl space. It implies they are about the height of your knee...but the height of any wall is a design or structural issue. If you mean the height of the curb, the piece of the shower that sits under the door, then I have some help. Curb height here in Sonoma County is required to finish out 2" higher than the top of the drain. The reason for this is that if the drain clogs, you probably have enough time to finish your shower before it overflows. With a vinyl shower pan and clamp in drain, we figure we need to be about 5 1/2" to our rough framed curb (before tile prep and tile) for a 48" wide shower. Consult with your tile setter (who should be building your shower pan as well) to get the correct height).