3mm. I think I'd like to go smaller, but I use that same size all over the boat and I usually buy it by the spool. A spool of 2mm would last me a lifetime just using it on trapeze parts.
The 2.5mm (7/64") Amsteel is fine for trap lines. Just be ready for it to have some initial stretch. My a-cat lines stretched about 4-6" before taking a final set.
