Heya,
Mbu puffers are the largest puffer available I believe. They need a minimum tank size of 300 gallons, otherwise they won't be able to turn properly. They normally won't tolerate any other fish, even other mbu's. They are fully fresh water unlike some sources say and definaetly need MUCH MUCH MUCH more than 50g like some places suggest. They can grow much larger than 30" even and like all puffers are VERY messy. Check out **************.org for more information.
Fahka don't need 300g, 125-150g would probably be suitable. They like to bury themselves in sand. At largest they are 17", usually being a bit less in the home aquarium.
It's good to add some salt for all puffers to help ward off diseases. They are great fish in general. I keep dwarfs right now(not exactly Monster LOL)and ordered a Figure 8. They do need to have shell based food though to keep their beaks down. Either way you may have to end up sedating the fish and clipping it's teeth. More info can be found on the site supplied above.
Good luck with whatever puffer you choose. They are rewarding pets, and yes I do use the word pet for them. They seem to love to people watch, even in fish stores when I see puffers they always seem to be staring at me. They seem to beg for food and nothing is more satisfying then seeing a puffer take down a snail, crush it with it's beak(fused teeth) and suck it on up.