There are different types of classes and the way they operate often varies slightly from one show to the next, so your best bet is to contact the centre to find out. You can read more about the Trailblazers showjumping league here, and scroll down to the bottom of the page for PDFs of the rules.
http://www.trailblazerschampionships.com/information-showjumping.php
Saying that I should think it's most likely that it will be a standard format class - everyone jumps the first course (which isn't against the clock BUT there will nearly always be a maximum time allowed), and then all the clear rounds go through to the jump-off. The jumpoff is usually a shortened version of the existing course, sometimes with the numbers mixed up a bit, for example your jumpoff could be fences 2, 4, 5, 6, 7 and 9 or something that requires a little more planning on your part, for example 2, 6, 7, 4, 5 and 9.
Normally the numbers for the jumpoff are available while the class is being run and they will either be written up on a board somewhere or on a piece of paper for you to collect from a judge or steward. If you get a clear round then it's a good idea to get hold of the jumpoff numbers as soon as you can so that you have a chance to work out your route.
Whether or not the jumps get bigger for the jumpoff depends on the competition so you will need to check with the venue. But they do usually go up a little bit, in unaffiliated comps it's normally only 5cm or so. I know it sounds intimidating but to be honest it shouldn't really be a problem - because one of the principles of preparing for SJ is to train at home over HIGHER jumps than the classes you compete in. So if you plan to enter the 65cm class you should be practising over 70cm+ at home - practise for both the horse and for you. That means if you do get into the jumpoff and the fences have gone up, your horse will be perfectly capable of doing them, and there's no need for you to have a panic attack!