usualy first year courses are pretty flexible, check with the uni you're aiming to go to that you can enrol in mining engineering and transfer to petroleum if need be. They're pretty much heading down the same path, and i would suspect that ypu can transfer at any stage. Just make sure you do courses that are applicable to both of them.
i.e. i've been doing geology, but can transfer to geoscience, environmental science, or geography pretty easily, just because of my subject choices.
As per class sizes - it really depends what uni you are at, but at ANU 1st year classes will be packed (over 150), then they thin down pretty quickly (60 in second year, and 20-30 in 3rd). In your first year you do 'generic' science courses, so you will be in a large cohort (don't be suprised if you are with 300+ people).
Finally, as per you "which would be the best uni to study" question. You should research this yourself. Go on the AusIMM website, click the students section and look at AusIMM certified university courses. This should be very useful in getting into the mining industry.
(PS - join it too - only $11/yr and you get invited to heaps of events/get 4 glossy magazines a yr)