Shooting a model on seamless can be tricky if you want to fit a whole model on it, especially when he/she is jumping. But there is a real easy technique. Although it needs some room. 

As you probably know all lenses behave in a certain way. Wide angles show you a lot of the area and longer lenses compress and also show you less of the background. We also call this angle of view. Where wider lenses have a wider angle of view. 

With this knowledge it’s very simple to fit a model on a background 😉
Just move all the way back as far as you can and use the longest lens possible to shoot your subject. If you can move away far enough you will see it’s incredibly easy to fit your model on the paper. Of course you can also play with the distance to the background.  

But remember the closer you move the model to the background the more chance you run on shadows on the background. So make sure you choose a balanced position if you can’t move back far enough.