Head on over to looptics.com and read his article about mirror cell supports. It'll get you thinking and result in something similar to what you see pictured.
The bolt sizes you can kind of choose - i use 3/8" on mine, though they probably don't need to be that big for support reasons on a 12" mirror. Still, they are beefy and readily available. 5/16" would probably work. 1/4" seems too small to me but I don't have an engineering basis for that impression.
The mirror cell support points are generally recommended to be .75-1" in diameter. DO NOT USE BOLTS. They need to be made of delrin or nylon. You might find a nylon bolt you could use I suppose, but you'll need to flatten the head as they often have a nib from molding and/or some writing embossed on them. The thickness of the head needs to make sure it clears any other hardware that is attached to the rocker arms, so the mirror is ONLY sitting on the support points. You can shim it up with washers between the support points and the pivoting bar if necessary.
Whiffle supports as shown are discussed in the article above, but generally are 45 degrees apart . The center of each whiffle is 45degrees off the vertical axis of the mirror, and the points are 45 apart from each other. The upper left support is 67.5degrees off the bottom center of the mirror, and so is the one one the right, so the total coverage is somewhere around 135degrees.
For aluminum thickness, on a 12.5" mirror this may be overkill, but I like to use .25" thick flat bar for the sides, and 1" square tube for the...tubes. 1/8" wall on the tubing, so it has some meat to drill/tap if you want to. The pivot bars should be at least 0.5" thick, and 1" wide. This gives enough meat to drill for the pivot hole and bolts to hold down the support points.