Hmm...dimensions? dpi? When you are creating either letterhead (no "s"), brochures/pamphlets, and envelopes, dpi has no relevance. That is assuming you are using publishing software like Indesign or Quark Xpress as opposed to image/vector programs (Photoshop, Fireworks, Illustrator, Freehand. If you are using the appropriate software, the selections will be there for you to choose from. Then you simply place the artwork where you want it to be.

If you are using image/vector programs, simply create the artwork. The print shop will handle the rest. Unless you are using an online printer, which will have specific instructions on how to set up the work.

Standard dimensions for what you have asked vary, with the exception of letterhead. That is, 99.9% of the time 8.5"x11". Brochures/pamphlets and envelopes come in various sizes. That all depends on what you want to do. Envelopes have various standard sizes, with windows, no windows, large, small, etc.

I can tell you though, when I work on print pieces, I rarely send finished work to a printer that is not Indesign or Quark. I used to work at one a while back and always had to fix crap that was sent to us from Photoshop or Illustrator. You can't run plates from those formats and we always had to place the work into Indesign or Quark.