No one thought very much of 14.1hh Teddy O'Connor until he strolled out onto the course on Rolex Three Day in Kentucky and rocked the cross country course in the PanAm games...he was an Olympic short-listed pony. Size doesn't really matter as long as you have the scope and heart.
If your mare can't find her spots, she isn't just fast, she's unbalanced and most likely green over fences. A balanced horse will take a jump three ways, short, long or in-stride without changing the pace of the canter. Jumping is essentially stride control and if you can't ride the canter forward, you can't change the stride.
It may be a good idea to take a step away from jumping and start at the basics. If you aren't sure quite how to get where you want, find a trainer who is experienced in eventing who will have a background in Dressage and cross country. Make sure your trot work is good, your horse is balanced, she's into your hands and is accepting of contact. A horse who doesn't accept contact either roots out of it or draws away and both are a recipe for disaster for jumping and even more so over cross country fences. Work on lateral bending, shoulder-in, haunches-in, leg yields, etc...the best jumpers (in hunters, jumpers or eventing) usually have very solid ground work and a lot of times even hunter riders school second level Dressage movements. Circles are so important in schooling and by changing the size of the circle you make your horse think, help her balance and slow her down in some cases. If you work a lot on lateral movements, you can use them as tools when your mare gets too fast. Asking her for a shoulder-in should make her stand out to her hand, her back should soften and she should slow down if she is being unbalanced. Having this background and these "tools" to use will make cantering and jumping so much easier and enjoyable for both of you.