Size IS important
A question that eventually pops into the minds of home flight deck builders relates to size. Because, after all is said and done, size is important. Having decided to build a perfect replica, one, of course, is determined to find the exact dimensions for the panel, the pedestal, yoke/side-stick, etc.
Or is it?
Let me suggest that this may not be necessary.
Unless you have a lot of cash and a very understanding significant other, you'll be making a number of substitutions using items that are not the same size as the flight-rated items. You probably won't have a true, flight rated multi-function display. When you park a computer display behind an instrument panel cutout, you'll find that you can't mount other items as close to the cutout as is done in the real panel. There will probably be a number of things you won't have. Right from the get-go, you'll find that you'll have to make adjustments. So, don't get too focused on precise dimensions.
Certainly, SOME dimensions are
There are some dimensions, however, that are absolutely critical. For instance, how much room do you have for your simulator? And, if you're planning on building your simulator inside, (and who isn't), pay close attention to the width and height of the doorway you will eventually use to get the pieces out. Let's not add a Flight Simulation chapter to the "Boat in the Basement" book.
This calls for some significant up-front planning. (I actually have done some after-the-fact planning for CYA reasons. But as this is a hobby we can skip it here.) Using pictures, posters, books, etc., (check out Airliners.net, Bob's Aircraft Documentation, and Historic Aviation) and honoring whatever constraints your particular situation places on you (http://check.yourhome.dude), design a cockpit layout in the style of your preferred aircraft.
Dimension sources are limited, but do exist
Notwithstanding the comments above, it's still nice to get a feel for the actual dimensions. Unless you've got some really good friends in the aerospace industry, you may find it rather difficult to find them, so you're going to have to be creative. Some builders have been quite successful measuring relative dimensions off pictures. Knowing the actual size of some of the flight instruments can provide you with a reference for calculating others. Tweak the panel layout to accommodate the monitor(s) you will use. Tight for space? Consider building only the pilot's half of the flight deck. Planning on getting fancy with a projection system just as soon as your tech stocks recover? Make sure you leave room for the projection screen.
You can find a wealth of Boeing and Airbus dimensional information on Andreas Toepper's excellent site where he has posted numerous cockpit photos incorporating a millimeter scale. You will find good information on B747-400 dimensions at cockpit.varxec.de. Juan Cordon has assembled a number of dimensioned drawings of the B737 on his site. Mark of MarkusPilot.com has also assembled a useful list of B737 dimensions. You should also check out the archive area of the Simpits site.
If you are interested in a Cessna cockpit, there is a wonderful gallery of photos of a C172 cockpit here.
Dimensions for Embraer ERJ 145 can be found on the ERJ Made Easier website.
You can also look on various avionics manufacturers' sites. Equipment brochures often have dimensioned outlines. Visit your local library (because the darn things cost so much) and look for any of the Jane's series of aviation titles in the reference section. The Jane's Avionics annuals in particular are good resources. Check out the detail books the model airplane buffs use. If you plan on using any replica panels made for the recreational market, you can get dimensions of those panels from the manufacturer. Finally, surf a bit and find personal sites of hobbyists working on your chosen bird. The simpit links page here has extensive lists of links to builders. Chances are a polite email will turn up some helpful info.
If you're using the scale-from-pix approach, there are a few dimensioned drawings under this page that may be of help. The links are at the top left of this page. Also, there actually are some standard dimensions in the aviation world. A "standard form factor" refers to an industry standard avionics slot that is 6.25" wide and 2.6" high. You will most likely see this in general aviation radios. Another useful tidbit is that a great many CDUs are 5.75" wide. This is a common width dimension for panels in the center console of commercial aircraft.