1.75mm, almost no delta uses 3mm and many cartesians are moving to it these days.
With a .4, you will be pushing it pretty hard, but it will work. Just make sure you don't crank up the amps too high on the driver and I wouldn't go any smaller on your head size.
It can still get pretty warm on a long print. One thing I like to do on all of my extruders, is if you have an old motherboard, take the chipset heat sink and strap it to the extruder motor (with heat sink compound and zip ties). It's cheap, and quite effective.The ones from P4 and Core2Duo motehrboards are usually just about the perfect size.
With a .4, you will be pushing it pretty hard, but it will work. Just make sure you don't crank up the amps too high on the driver and I wouldn't go any smaller on your head size.
It can still get pretty warm on a long print. One thing I like to do on all of my extruders, is if you have an old motherboard, take the chipset heat sink and strap it to the extruder motor (with heat sink compound and zip ties). It's cheap, and quite effective.The ones from P4 and Core2Duo motehrboards are usually just about the perfect size.