What limits the size of a fish that can go in a tank?

Is it a space issue where , they need enough room to roam?
Is it a space issue concerning chemical/toxi build up, such as ammonia/Nitrates etc?

 

Both, along with available oxygen

 

I would say both, but i think mainly its the room to swim around and grow to a adult size.

 

Def both

 

both but i agree biggy is oxygen k+

 

All of the above. The tank needs to have enough biological activity to keep up with the bio load of the inhabitants otherwise things go downhill fast. The fish needs room to "stretch its legs", and you need sufficient surface area and agitation for good oxygenation of the water.

Another good rule to follow is always consider a fishes adult size when purchasing. Even tough your intention is to move it to a bigger system, we all know good intentions often get overlooked or ignored and the fish becomes cramped. Don't put a grouper or tang in a nano reef no matter how much you want to. If it doesn't fit the inches per gallon tank size rule at adult size, whatever that is, don't put it in there even temporarily.