I want to try making epoxy weld beads on my models and from what I understand I should take a few brass tubes and grind them down and then a chamfer like adam wilder did in his metal dvd. My question is what size brass tubes should I get in order to make the tool? Not only diameter but also the tubes wall thickness.

Depends on how big you want the weld to be. Wall thickness is not that important beyond being able to fit it into an internal angle but I think the one I use most is under 1mm internal diameter. 1mm will give a weld 35mm wide in 1/35, which is quite big.

Not sure how set you are on making your own weld beads, but I can strongly recommend these: http://www.armorama.co.uk/review/12427