How to size your upper jaw- with sliced cheese.

This is pretty easy, just assemble the parts- sliced cheese, a ruler and a spatula. The spatula should be able to fit into your mouth. Otherwise you can use a piece of stiff cardboard cut approximately to size.

Get some typical sliced cheese and a small spatula.
You can trim down the cheese to make this easier.
Push it all the way back so you can press upwards un the spatula forcing the cheese onto the teeth.
To make the cheese impression easier to read, you can take your fingernail and remove the cheese on the outside of your teeth. Do this while the cheese is still in the mouth. Here, you can see the cheese impression shows my mouth is approximately 6.5 cm wide.
You can also measure the length of the dental arch. This one shows about 5.0 cm.
This is an impression tray for the upper. The sizes of the dental mouthpieces correspond to these trays. This tray is for use up to about 6.5 cm wide. We’re mainly concerned with the back teeth, since that is where the traction force usually goes. If your jaw is under 5.5 cm wide, you will probably need to special order an XS mouthpiece.
A medium tray is usable for an arch from 6.5 to 7 cm wide.
A large tray is good from 6.8 to 7.5 cm wide. If your jaw is wider than 7.5 cm, you will need to special order a XL mouthpiece.
These are the three main sizes of mouthpiece. L is on the left.
Here you can see the width of the large mouthpiece. Inside measurement is about 7.5 cm.
This is the medium mouthpiece, inside width is about 7 cm.
This is the small mouthpiece, inside width is about 6.5 cm.
You can see here that sometimes the back teeth, in this case the wisdom teeth, are flared out. The outside portion of these teeth, next to the cheeks, will not be in contact with the silicone pad.
This arch would be fine with a small mouthpiece.
This is my silicone pad after about a year of use. I prefer the pad to be just a bit wider than my dental arch. This fit is quite comfortable. After a couple of years I will replace the silicone.
Keep in mind I use a hygienic plastic bag (sandwich bag, etc.) over this during use.

This is the side view of my silicone pad. You can see the material is above the metal rim. This keeps my teeth away from any metal.