Margo,
My largest size needle is a 4,0mm. Mainly I knit my sweaters on 3,0mm and 3,5mm. The majority of my knitting is sweaters, mainly Norwegian patterns or guernseys. I only use Addi Turbo circs, except mittens and socks (1,75mm-2,0mm), where I use short dpn Addis. I cannot imagine knitting with a needle larger than a 4,0mm.

My friend decided to knit and her smallest needle is a 15.