Lets take a step back here, there are several things:
Q: Why are big ships a bad idea?
A: There are some problems such as getting into docking slots. Mostly large ships should be ok, but the game was initially built around cobra sized ships, so there are no guarantees weird things wont happen if you go really big. You would expect very large ships to be extremely tough which
might unbalance the game. You would also expect large ships to have large cargo capacity, and that doesnt fit very well with the commodities market in oolite which is limited in amounts you can trade.
You dont actually get much from size, you dont get to fit any more weapons, you just become easier to hit, and 200t of cargo is more than enough for any normal play.
All in all large player ships are possible, but at least in my opinion they dont fit the game well. People are free to disagree
Q: What is the largest possible ship that will work properly in the base game?
A: A ship the height and width of a normal docking slot is the biggest that you can expect to work fully and not require work-arounds (there are things like using the auto-dock of a docking computer, but thats a bit of a cludge).
Q: What is the largest ship actually available in the basic game?
A: I think its the Anaconda, and I think even this is hard to fill at the commodities market so much of the space tends to be "wasted".
Q: Whats the largest ship you could create?
A: It is essentially unlimited, but as stated before, you need work arounds for docking etc.
Q: What is the largest OXP ship?
A: I dont know, if you include RS there are some huge ships. I suspect something from Amen Brick.
Q: Why can't I fly a Behemoth or other ship?
A: When a ship OXP is created, the author decides what roles they think it should have, and if it should be available to the player. Some ships are intended to be police, military, or otherwise not available to players. This can be "fixed" by changing the OXP, or using RS/OSE but its worth understanding that if a ship isnt available, then its a deliberate choice of the OXP author. As an example Charlie created an "unknown" alien race called the Orb which are sometimes encountered, and obviously their ships are not purchasable.