Seems that there aren't many free online programs to create posters from a letter-sized document. Here and there, I found a few to download, but it is much safer to work on the site itself. So, if you can't afford to make 20x30 or 24x36 poster prints, do a little creative cutting and pasting and you can make you "poster" as large as you like. (I print on photo paper, so it's a sturdier sign.) There is a video below which shows you how to put together your poster.
Block Posters- I've used this one many times and like it
Easy Poster Printer- XP, Vista, Win7 & Win8 download (no Mac)
Faster Poster- download .exe file for your Windows computer
Posterazor- cuts your image into pieces and gives you a PDf file to print and cut/paste together
Rasterbator- upload your file and choose how many sheets wide it will be
Block Posters- I've used this one many times and like it
Easy Poster Printer- XP, Vista, Win7 & Win8 download (no Mac)
Faster Poster- download .exe file for your Windows computer
Posterazor- cuts your image into pieces and gives you a PDf file to print and cut/paste together
Rasterbator- upload your file and choose how many sheets wide it will be