Design to the page size for letterheads:

US: 8.5" x 11" (letter)
UK: 210 x 297mm (A4)

If you want your design to be full-width and/or height remember to include a few mil extra for the bleed - also remember that this will cost your clients more money at the printers.

Envelopes are a little larger than the letter width and a little over 1/3 height but I don't remember the exact sizes off-hand... Google will help you here. You should actively avoid up-to-the-edge designs for envelopes as including a bleed is not an option and would require the end product to be custom built.

Pamphlets vary so there isn't a single preset size.

300dpi is good.

Other things to remember include working in CMYK and keeping colors down to a minimum to reduce print costs. Look to use Pantone colors to maintain brand consistency.

When creating Letterheads include a second, lesser version for continuation sheets.