For a 16" wedding cake would you use a 20" cake drum (thick foil covered board)  I thought I read once to allow 4 inches for the bottom board for border and ease of transportation

that gives you 2 inches all around. sounds right. I like a smaller board at the bottom. If it's a tall cake, keep in mind that it will be uber heavy at that size, so you may want to assemble on site

AYes, that sounds about right, especially if you have flowers or any decorations that might need room on the bottom tier. Definitely what ellavanilla said, super heavey so make sure you assemble on site.