Mrcharly,
I just had a look on the Miller Resources site because I thought there would be a relevant calculator on there but can only see calculators or look up tables for the selection of current once you have decided on the rod. I guess the choice of rod size in general depends on the type of weld, how many passes etc. Personally I'd choose 3.2mm rods for 4mm and 2.5mm for 3mm - I generally have a go on a bit of scrap to sound it out before starting on the actual job. They say bigger rods are easier to learn with if you are just starting out. I recently bought some of the SIFtrode 6013 rods from the forum shop and they seem pretty good. Here is the link to the Miller site for general info, tips and advice, just in case you haven't already stumbled across it.
think i would go for the 2.5mm rod personally especially if using on 3mm, less chance of burning too much of the outside edge away.
approx 70-75amps bury it right into the corner and weave it approx 2mm
let us know how it goes