The type of candle you use depends on what you're using it for.
Wax does not have to cover the jar for it to be effective.
A popular method is a taper candle rolled in oil and/or herbs and burned on top periodically.
Personally, the only time I put a candle on a jar is to read how the work is going (ceromancy). I tend to shake my jars to keep them working so I don't need to put a candle on the top.
If you aren't reading the wax to see how the work is going, the mini candles are fine if that's what you're comfortable with.
The magic isn't in the candle, it's in the jar ;)