how big should the eraser end of a pencil has to be?

Just a normal pencil, held in the lips, no teeth used.

Don`t overdo it, just try a little longer each time. move it slightly up and down with lip tightening. Some people say to try hold it stright out flat. This is too hard, and not sure of the benefit apart from tiring more quickly.

There`s a lot of differing opinions and different techniques, read as much as you can, and talk to as many musos as you can to find the right advice that suits you.

I don`t think it is a miracle cure. Just a help.

should my lips shape like a kiss? or should it be covering my teeth?

Your mouth/embouchure should be close to normal for playing trumpet.

Say "Iffff", and plae the end tip of the pencil between the lips. The front top teeth will slide back slightly to fit the pencil onto the lips. Now the trick is not to use the teeth at all for support. (my teeth in fact can close behind, and don`t touch the pencil at all). Use you lips and embouchure to keep the pencil at an angle, you should start to feel a slight amount of work in the corners of your mouth almost straight away...this is working the muscles...almost like isometric exercises. Again do not overdo this work. Repeat is better, and then go a little longer as you get used to it.

If you move he pencil to a higher angle, like straight out in front, you find your jaw may be moving, but the embouchure should be working harder as well.

what can i do to help my arm pressure, is there anything like the pencil exercise, except it helps arm pressure