1)    What size painting should I choose? It depends on where you or the couple would like to hang the art.  I love working on a 24x36.  It can hang vertically or horizontally without taking up too much space, but large enough to be the focal point of the wall.  This painting will ultimately be an heirloom, much like those beautiful, old black and white photos of your grandparents, this too will be looked on by generations after you.  
2)     What kind of substrate do you paint on?  I use Gallery Wrapped Stretched Canvas. Sometimes I stretch it myself and sometimes I order pre-stretched canvas. 
3) What medium do you use to paint? I utilize mixed media which may include, but not limited to oils, acrylics, liquid inks, and different forms of dry sketching media.