QuickSiri is a tweak that does two different things. It bypasses Siri's normal half screen size, and takes it right to fullscreen when you invoke it. It also claims to speed up Siri, and Spire on older devices, quite a bit. It works well, so if you prefer the fullscreen Siri, you may want to check this out.

I did not make this video, nor am i responsible for the contents of it.

This tweak is available for free on the ModMyi repo.