What is the standard scientific conference poster size and quality?
Conferences should tell you what poster size you should use - the ideal poster size is 'one that will fit on the poster area assigned to you by the conference, without being too small'. 

Normal paper is a bit lightweight for travel, most of my conference posters have been printed on thicker paper (which may have been photo paper; generally, I've just gone to the campus printing facility and asked them to print the poster, with no input from me as to what material the poster should be printed on). I've seen a few posters printed on flex, and while it's something of an unusual choice in my field, I can see the benefits in storage and transportation.