Well, considering there were a lot of knights (see list here: http://en.wikipedia.org/wiki/List_of_Knights_of_the_Round_Table )
how big was the round table?
According to wikipedia, almost 60 knights are mentioned explicitly by name somewhere. The table itself was massive. Lore has it that the table was bigger than this though. The common estimate isn't that Arthur would just get a couple of knights at a time in for conference, you go hard or go home. Arthur's table sat 150 knights! This meant he had a LOT of dinner guests and the need for a lot of space for feasts and whatnot. Assuming this table is for hosting dinner, we will give each knight a decent amount (yet not unreasonably large) 3 ft of space at the table per person. By simple multiplication, the circumference would easily have to be around 450 ft. Which would translate to a 143 ft diameter. Now no one ever technically said it was a circle, since as long as the table didn't have points it was round so technically it could have just S curved everywhere, so my specifics might be invalid. However, assuming it isn't a metaphorical "table of allegiance" or something lame like that, it would have been a really big table.