For a letter-sized document that will be folded into three panels (Letter or Tri-Fold), a #10 envelope should be chosen. This envelope is 4.125" x 9.5", and can fit several folded sheets comfortably, as well as smaller return envelopes (a #9 or #8). For flat letter-sized documents that will not be folded, a 9" x12" size should be picked. Again, this size allows for multi-page documents to fit comfortably. If the documents will be inserted into a 9" x 12" Presentation Folder, then a 10" x 13" envelope will accommodate the folder as well as the documents inside.