Measuring the flame
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Flame height is one of the properties ASTM F2417 addresses, and it is the easiest thing to observe and the easiest to wave away.
Height is a safety property
A tall flame is a candle putting more heat into the vessel, into the air above it and into whatever is nearby than it was designed to.
It is also the property most affected by things you changed for other reasons: more fragrance, a different dye, a bigger wick chosen to fix tunneling.
How to measure it
Against a ruler held beside the candle, not against the vessel. Read the steady height rather than the flicker peak, after the flame has settled for several minutes.
Measure at each cycle, because flame height changes as the candle burns down and the wick sits deeper in the vessel.
Record a number rather than an impression. "Tall" in your notes in March means nothing in June.
What a bad flame looks like
Tall and thin, reaching: usually an over-large wick or too much fuel being drawn.
Dancing with no draft: unstable combustion, frequently a fragrance load the wick cannot handle cleanly.
Two flames, from a split or mushroomed wick.
Guttering and low: under-wicked, drowning in its own melt pool.
Any of them in a draft-free room is the candle telling you the system is wrong, not the room.
What changes it
Wick size, obviously, and wick type: a cotton core and a paper core of nominally the same size burn differently.
Fragrance load, which is fuel. Raising it from 8 to 10 percent can move a flame noticeably.
Dye, which adds solids.
And the vessel, through how much heat it returns to the pool.
Which is why each of those is a retest rather than a tweak.