I would like to know if I would need a 1.5m x 2m Backdrop, what's the size of canvas I've to create on photoshop?Thanks!
There are a couple ways of doing this. Firstly: How are you printing this? How close are people supposed to be looking at this backdrop?
When you create a new document you can choose what size you want in several different units. Simply change to millimeters or centimeters and plot in your size, then choose a fitting DPI. 300 is for photo realistic quality, however once you get to those sizes you can go down in quality due to increased viewing distance, 150 is probably decent.
Alternatively you could make your file in a 1:10, ie: 15x20cm and then ask the printer to produce at 10 times the size. Doing this you certainly need a higher DPI.
Lastly though, what are you making? This sounds more like an InDesign job than a Photoshop job.