Standard size double doors are general 5'-0" (2 2'-6" doors), 5'-4" (2 2'8" doors), and 6'-0" (2 3'-0" doors). Appearance of double doors or single doors with sidelites is totally dependent on the person and the style of home.

Depends on where the double doors are located. Front Doors in my area are most often approx. 72x80. However rear doors such as French Doors can range between 48"-68"x80 with the most common size being 64x80. The largest sidelite offered by most door manufacturers is 14" but there are some styles available with 16" S/L.

As noted, which looks best is a matter of personal taste. I've never really seen any benefit to a double door. My personal preference is a door and 2 sidelites.

I agree with skydawggy, also double doors are prone to leaking especially in a front door due to the fact that they are normally supplied with a fairly low profile sill as opposed to patio doors which are generally supplied with taller saddles.