For models, 9x12 with full bleed photos or tear sheets is the accepted standard in the major markets. I don't think you could go wrong with this.
I understand 8x10 black and white is the standard for actor's headshots (not really a portfolio per say).
For photographers, no real standard. I use 11x14 with full bleed photos. My latest one is a Prat Pampa in red leather.
http://www.pratfab.com/US/Produits_s...ampa143147.htm
For interviewing with gallerys, your portfolio is matted photos in sizes appropriate for hanging, say 11x14 or 16x20. These are contained in a portfolio case, so the gallery owner can lay them out and see how he would show them.