are you planning on going fully salt or staying brackish?
http://www.**************.com/forum/ug.php/v/PufferPedia/
go there and search the different brackish and saltwater puffers. the profiles have minimum tank sizes and how big the puffers get. probably be best off going a little on the larger size so the puffer doesnt fit in his mouth, but that still might not guarantee him not trying to eat it. if he tries to eat it and gets the toxins in him your eel WILL die. as long as your willing to risk it, i would go bigger puffer, but most if not all bigger species of puffers seem to get really big and need something along the lines of 100 gallons. just check out the puffer pedia and browse around for a little bit to see if you see something that can be kept in a smaller tank like that but still get a decent size