A small spell or ritual that is quick to do and needs only a short burning period allows for a tea light to be used, but they only come in limited colours, so colour correspondence is more difficult to do.
Those rituals that include the burning of a plain or coloured candle for a longer period of time are more suited to using long candles, as they tend to come in more colours for representation and can be burned down gradually over many spells.
It is up to you which you prefer to use, I prefer the longer candles as I can carve sigils, symbols, anoint them easily, etc.
You can use whatever candles you can work with. But it does depend on the activity. Tealights only burn for about an hour to an hour and a half. You can still anoint and inscribe on them, but then again, it does depend on the working.
I use tealights as illuminator candles, or for a very short working
This is your choice I suppose. However I would suggest to use candles made of natural organic materials. Moreover you should be careful on the color you will choose. There are articles on SoM that can guide you regarding candle magic. Also you should cleanse your candle before you use them (I prefer sea salt for cleansing).