I want to create and print a 50x100 cm poster. I tried 50x100cm,but that gives me a psd that is very large(almost 200 MB). What sizes should I enter for the canvas?
If you change your image size to 50 x 100cm, and you use a 300ppi resolution, it's normal your image will look very large on screen and the file will be heavy.
If your poster needs to be at that size, then that's the dimension you should use in Photoshop.
Depending where you get your poster printed, you can lower the resolution though:
1) If your project will be printed on offset printing and in big quantity, then you should keep the resolution at 300ppi (300 pixel-per-inch or 188 pixel-per-cm).
2) If your poster will be printed in small quantities on a large format digital printer, you can use 200ppi. An example of large format printing on digital: a pull-up banner.
In all cases, you should refer to the printer's website or file preparation guidelines; they usually mention there how much is the minimum accepted resolution.
If your computer cannot manage such a big file size then you can create your poster in vector, in Adobe Illustrator. Vectors are lighter than rasterized images (Photoshop) and they can be created at smaller size and then get printed at bigger size without any loss of quality.
Of course, if you add pictures in your Illustrator document... the file size will increase as well and these pictures still need to be high resolution and at the right dimension!
You can also use Indesign and only do the background of your poster in Photoshop. You can then import that background image in InDesign and add the texts and logos in InDesign instead of Photoshop; it has better performance for big projects, and the quality of your printed text and vector logos will also be better.
PS: Don't forget to add your bleed, if necessary...