I've found Etienne Muller's step by step instructions very helpful but I have a few questions.

Etienne's pics and instructions are excellent. If you have the time, you could profit from looking for other ways of building skegs and skeg boxes - some use a '2-rope- system vs cable. (Bjorn Thomasson has skeg info on his site, and there are others - of course a search here at kayakforum will turn up lotsa different examples as well. )For me, more info is always better - but I know it can cause information overload and confusion.
Having a good look at commercial skegs (YouTube ads and sites with info on skeg repair) can also provide insights.

Buying the KajakSport (?) skeg assembly is one good option, IMO. Sean Dawe has details of that installation.

1. Is there specific guidance on size (width & length/depth) of a skeg?

I think most folks use commercial skeg dimensions as a guideline, or dimensions from a reputable designer. Since they are adjustable, it's not too critical if the skeg is a small bit too large - most folks I know often paddle with 'part skeg'. If you ask for specific dimensions you can get some input here, I'm sure. (You need to specify if you are going to use a 'triangle' style skeg vs 'fin' - there are some of the latter style.)

2. Cabling system seems logical for raising the skeg; however, I"m unclear how the skeg would be lowered. Does the skeg drop rely on gravity? The design seems like it creates a "pushing a rope" scenario ... how does the cabling 'push' the skeg lever to cause the skeg to drop?

Yes, it is 'pushing on a rope' - the cable systems use stiff stainless sailboat rigging cable (1 x 19), not flexible wire rope. The pushing on a rope problem is the major deficiency of cable systems - most have a section of unsupported cable when the skeg is 'down'. Running over a rock or beaching without retracting the skeg can kink the cable, which generally requires cable replacement, since the cable will jam in its tube if kinked. Reg Lake has solved that problem with his very innovative design, which is used in the Sterling kayaks. Reg has been generous in sharing details here on kayakforum; I also posted some pics in my Frej build discussion.

The 'two rope' designs are simpler to build and easier to maintain, IMO, though a bit less precise for 'half'skeg' deployment.

Not to mention 'skeg vs skeel'..... lots of fun!
Another issue to consider is the 'pebble jamming in the skeg box' ....... plenty of fun..