I have had some people put radishes in a Bonsai pot, Matt put a watermelon in a Bonsai pot, even Bunnings put them sticks it Bonsai pots. That does not make a Bonsai of it.
Matt I believe this is a valid question as we have as many varied answers to this as we do to potting mixes.
Jamie, please be careful Azalea's are shrubs, as are many of the Melaleuca's remember they are described as Tree and or shrubs. Shrubs do make very good Bonsai.
My personal opinion for what it is worth is a tree and or shrub that has been styled by Man ( as in Homosapian ) to represent a Tree in miniature.
This must be done in a manner as to be artistic, and show no real sign of man"s involvement, which is what makes a good Bonsai stand out from a stick or radish.
In a Bonsai pot.
Size is of Conjecture because if a tree naturally grows to 15 to 20 meters in height at 2 metres it is a miniature of that tree.
In Bonsai culture, sizes are ambiguous, there is a starting size to most, overlap the next size up for starting such as Mame start at will fit in the palm of the hand by one Book to Mame size trees will fit up to 5 in the palm of the hand.
I have trees that are 4mm in size I can fit 4 safely. Now Shohin under 20cm in some books others 23 cm.
Bonsai Kia London UK's oldest formal Bonsai club. Rules the Shohin shall be under 6inches ( 15cm ) including pot. There is also a Shito size which is very small. Then you have Ippan which is from 45 cm to 1meter.
That is how I see Bonsai, as artistically, created pieces of Art from living tree's and or shrubs.
Cheers

Ps the management reserves the right to say rgestuffed.