This hat could not be easier or cuter. Use a soft yarn! You are looking for an approximate gauge of 4 stitches to the inch. You'll need one skein for any size hat. Pattern is written for a child's size. The yarn shown in the picture is no longer available. This looks best in a handdyed yarn because the color variations will hide the decreases.
You can adapt this to an adult size by casting on more stitches, always in multiples of 4, until you have the starting brim size you want. This hat has a 10" diameter opening when folded (about a 20" circumference) and is 9" from point to brim as shown. I *can* wear it - but it stretches kinda fierce - looks like a cone on top of a basketball! Most adult head sizes are between 22-24".
Cast on 88 stitches, placing markers after stitches 22, 44, 66, 88. Join, being careful not to twist work. Work in the round until piece measures 4". On the next row, begin pattern stitch.
Pattern stitch: *knit to 2 stitches before marker, k2tog.* Repeat to end of row.
Repeat pattern stitch until 4 stitches are left. Cut yarn, leaving long tail. Draw tail through 4 remaining stitches, turn to inside and weave end in. Attach a small bell to the top before weaving in the end, if you wish. (I did this but Alex bit the bell off when he was a baby.) This hat fit him at age 10 months or thereabouts and also still fits him now that he's 3, although it loses the conical shape when it's on his head because it stretches a lot. But not unduly.