There are two considerations: comfort and strength.
Comfort is entirely up to you. @Worndown has made the choice I would advise when it comes to comfort: larger is generally better, up to about 3/4". But it's subjective: my father, on his sailboat, uses 1/4" rope for one particular piece of rigging and has no complaints. On my boat, I use 7/16" rope for the same piece of rigging, not because I need the extra strength, but because 1/4" feels like it's trying to garotte my fingers.
Strength is less flexible: you need a minimum strength equal to 2x your weight, and more is always better. You can get that easily in a modern, high-tech line like dyneema, in a ridiculously small diameter, but it'll be very uncomfortable. Some hardware store line that you'd think ought to be strong enough is pathetically weak, even in larger sizes --avoid polypropyline, which is typically ridiculously weak.
@alizee has chosen pretty much exactly what I would recommend, as a sailor and a climber: a 12mm climbing rope has reliable strength (both materials and construction), a smooth surface for comfort, excellent knot characteristics, and a good balance of strength and comfort in its diameter.