How do I design my posters on Photoshop touch for ipad, to be A0 and export them to creative cloud at the same size???

A0 is pretty big if you go at a high DPI for printing. It's about 10000 x 14000 pixels at 300 DPI.

PS Touch can handle about 12 Megapixel image imports (which is about 4000 x 3000 pixels). PS Touch, interestingly, can create 4096 x 4096 pixel projects from scratch.

Adding to that, PS Touch always exports at 72 DPI. Take note that this is probably due to tablet hardware. Tablet hardware is "getting there" but it still has a ways to go before it matches its desktop cousin.