Steve: You hit a good point. We have put in a few over the years. They come in two parts, a frame, and the fixture body. Can't really say what the dimensions are/were exactly. We do all comm work, 99% is sheetmetal framing. 24" o/c framing usually has a rough 22-1/2" space. Catalog I have now says width 23-3/4"; mounting brackets fully adjustable over 3-1/4" range from within installed fixture. BTW, the 23-3/4" is the same as a 2x4 lay-in.
