I'd say don't get on her at first. Put the saddle on, see how it fits, and then lunge her. You don't NEED a round pen to lunge, its just easier lol. Just keep her working in a wide circle on the end of the line and see how she goes. If she shows discomfort from the saddle then its better she shows it without you onboard in case she bucks, especially as she's young.
Don't get on until you're confident she has settled and that the saddle fits properly. You don't want it pinching her and then she throws you because of a reaction to the discomfort. Good luck!
Satin Reign aka "Misty"... my life, my love, my everything.