"板腰宽度" can be translated as "waist width of the board" or more specifically, depending on the context (such as in woodworking, surfing, skateboarding, etc.), you might use terms like "waist width of the plank" (for wooden boards) or adjust the terminology to fit the specific field, like "waist width of the surfboard"（冲浪板板腰宽度） or "waist width of the skateboard deck"（滑板板面板腰宽度）.
In general engineering or manufacturing contexts, "waist width of the board" is a clear and straightforward translation.