I think 1-2 mils is cool enough. It puts them in something with decent cargo to start trading and exploring. But you got to know most of them will blew it on some fighter and will be doing missions forever.
I had my share of donations, its nice to see new players around, especially ones willing to learn and adopt. Back then when I played more, and when I traded I would pick up new players with me to show them routes or to pay them for scouting and such. Teach them about forums, about space about rp etc.
So for anon donation I wouldn't go over 1-2 mills.