OK, I went through this a year ago and I'll just be straight about it. if you have a newer machine, they do have more room. An older machine does have very little room. Your head absolutely will go in (I had an abdominal MRI). I did go in feet first.
I did freak out a bit but the guy doing it was really good. He gave me a mask to wear, which did help. He also kept his hand on my shoulder while he put me in and kept it there until I told him I was okay. I did start to relax towards the end.
When I relaxed towards the end I was able to peek under the bottom of the mask and see a slither of the top of the machine. I had an older and smaller machine. It is close to your face. At that point I felt okay about it and just didn't think about it too hard.
They do give you a button you can push if you need to come out and they will remove you immediately. It's not once you're in there you're stuck at all. They also talk to you through it.
If you can, you may want to take something to help you relax if you're anxious. In the end, they are very good at helping you. I do recommend the mask if you don't want things close to your face. You can be in denial then. Remind yourself that you really need to do this scan and breathe.
I think if I had to do it again, it would be easier and I'd know what helps me.