The classic fillet size description that I've heard many times is the fillet should be 3 or 4 times the wall thickness. So a tube with a 1mm wall would have a fillet of 3 or 4mm in size. This is smaller then what most do, in the area of a TIG weld size. I try for medium to small fillets myself.

Far more important then whether the fillet is 4 or 6 or 8mm is whether while finish filing the tube gets undercut, So besides the practice of laying down the fillet one should try to file a few also. You'd be surprised how easy it is to file down the tube at the edge of the fillet. One way to avoid this is to file only on the filler, easier said then done. Another reason to practice the filing is to gain motivation to make nice looking fillets which require less filing. (I have a friend that was employed in a short run production frame company 25 years ago. He hated to do the filing of others' brazing. There was no feed back loop to train the others to do cleaner brazing).

I also agree with Eric's suggestion to make brass towers or other lumpy shapes off a surface. I've made a heart out of brass for my honey. Andy.