depends what you want to achieve really.
you might want to take a look at the colour temperature of your headlight bulbs (aftermarket or not) and get LED's of a colour that will match that. Sometimes you may find this a bit of trial and error as they produce the light in a different way and cover different wavelengths. So if you think you haev found the right one, buy a couple of them, hook them up and give them a go comparing them to your headlights before you pull anything apart.
for the voltage, it depends on how you will hook them up. It's worth reading into the various ways of wiring them up (parallel, series) and see what suits you. parallel uses more wiring but can be simpler to understand. Remember the alternator works at 14.5V NOT 12V, this always annoys me when people use the value 12V and this is why people burn them out all the time. You need to either spec the LED's based on 14.5V or the better option is to regulate the voltage to 12V using some circuitry.
size wise it's dependent on your application, 5mm are by far the most common size however you will find there 3mm, 5mm, 8mm, 10mm, surface mounted of varying package sizes, etc etc.
brightness and viewing angle go hand in hand, if you go for a high brightness LED that has a very narrow angle it will provide a very bright but small circle when projected onto a surface. if you get the same brightness with a wide angle it will flood a large area but it will be less bright. it's your choice what brightness and angle you choose and again trial and error is key, remember, you will be limited what angle you can mount them at.
There is a truck load of info out there on the application of LED's and what it all means. If you get really stuck you can ask in electronics forums like allaboutcircuits but make sure you do not tell them it's for automotive purposes as they refuse to have anything to do with the car modding scene for "legal" reasons. tell them it's for a school project to simulate a car headlight and isn't for a real vehicle.