How large is large?  Is this a mask to fit your face, your entire head, or something else?

One way is to form your dragon of oil-based clay on a flat wooden board, then apply papier mache as strips in as many layers as you think necessary for the size and strength you need. The smoother you keep your layers, the less sanding you will have to do before painting (etc). Ridges, scales, horns and such can be added as needed.

Sue

For something that large, I would think maybe a basic form of multiple layers of PM laid over a form (maybe an exercise ball?), any necessary openings cut into it when dry, then build up the form with pieces of styrofoam with glue (foam glue is good, but slow; low-temp hot glue is faster). Then layer over all with PM, decorate and seal.

You could use poultry mesh, but the cut ends of the wire can be rough to deal with. And painful.

If memory serves, there is a book called '3-D Wizardry: Design in Paper (sic) Mache, Plaster and Foam' by George Wolfe that showed how to make dragon.

Sue

The method used in the Chinese Dragons is to make a frame of flexible bamboo type wood.  Hazel would be a good one normally easily found in the UK.   Bend these into shape and bind them together.  Take sheets of paper (packing paper would be good - strong and malleable) and glue it to the frame.  Add more detailed shaping with screwed up paper held on with masking tape and then layer over the lot and paint.
DavidO