How to build a screen canvas?
If your heart is not set on a silver screen, I had good results using blackout cloth - this is white, rubber-impregnated cloth that is used for curtains.  I got a swath that was 48 inches wide - 1.21 meters, so a bit smaller than what you're looking for, but the largest they had available at the fabric shop.  By using a single swath, I avoided seams in the screen.  I made a 2.35:1 screen, so the final result was about 117cm x 275cm, which is really the largest screen my room could have handled.

I built a wood frame, stretched the fabric over it, and stapled it in place.  Four years later, it still looks great, and the total cost was under $100.