i would say the puffer is a little under 2 inches and the eelis about 9 inches. i'm not really sure about long term tank mates because of tthe eel mostly. what will prob have to happen is us get him into marine and set up a big salt water. we only have a 5.5g salt now and he's definitly not fitting there!
we got the eel first and the guy told us he's freshwater but on doing research he's obviously not. i think they have babies in mostly fw and then move to the sea. but since we had to set up a brackish tank it was a good reason to get a puffer. i'm sure we'll have to start getting stuff for a big sw tank soon. we could never afford it all at once so we'll by pieces at a time until we have enough to set it up and then get live rock. so the puffer will prob end up having the tank to himself, we're just keeping an eye on them both for now. they just ignore each other mostly, the worst i saw was at first at the first feeding time the puffer took a snap at the eel, the eel took a snap at the puffer, then the puffer darted in and nipped him and the eel ran away :o now they can swim right by each other and don't care. i just try to put food in both sides of the tank just in case. the eel was actually quite ceap. eels around here are very expensive but he was only 14 or 18 i can't remember. i was surprised.