When you take your licence, do the longest course you can afford prior to taking your test to give you maximum experience on the bigger bikes.
My local test centre uses cbf500s which are pretty heavy and you need to be pretty competent on a 125 before they will let you use the 500s. Most of the bike school machines are tourers and do not have the same grunt as a sportsbike, so bear that in mind after passing your test as its one thing to ride a cbf500, and another to have a go on something like a cbr600, r6 etc.
As long as you can ride the larger bike safely and have good control when riding at slow speed, you should be fine.
The bike test is going to change in March and become a lot harder and more expensive, so try to book yourself in as soon as you can.
Kit yourself out with the best helmet and protective gear you can, you cant put a price on your life!
Before buying a new bike, make sure you can fit comfortably on it with your feet able to touch the ground, because most new riders will take a spill at some point and youll need to be able to either stabilize the bike or at worse, be strong enough to pick it up from the floor!
Good luck, be confident and listen to your instructor and youll be fine!