Looking at this pic -
http://www.voxshowroom.com/uk/amp/ac10_hood.html you should be able to guesstimate the chassis dimentions. I laid out similar components and measured 13" wide, ish. I'd say the top was 8" or thereabout deep, depth of bottom chassis 1". Top chassis same width 13", 3" deep. Your bog standard chicken head knob is 1 1/4" across, 3/4" either side = 2 3/4" to 3".
I've seen a measurement of 20", wide, but that's daft, the cab will be built around the speaker, so therefore spkr is 10" x 2 = 24" total max add a bit for clearance.
But as your building a clone, you can make it better and whatever size you want using the original as the starting point.
Andy.