The question how to choose a size for printing comes up regularly on sitepoint. For printing you should choose a document that has a resolution of 300 dpi (dots per inch). That will give a good result. For artworks you will need a higher resolution, but for that you would talk with the printer who will print for you.
For all documents that you want to print, you must multiply the inches x 300 in order to arrive at the right size. Take into consideration the margins that the printer requires for your layout. Margins are the edges of the paper where the printer can not print.
Most printers will need:
Top and Sides: 0.25 inches or 75 pixels margin
Bottom: 0.56 inches or 168 pixels margin
Here are the most common sheets of papers in use, their dimensions in inches and in pixels. For the layout on each of the different sizes of paper you must deduct the margin when you work your design.
US Letter: 8.50 inches x 11.00 inches or 2550 pixels x 3300 pixels
US Legal: 8.50 inches x 14.00 inches or 2550 pixels x 4200 pixels
A4: 8.26 inches x 11.69 inches or 2478 pixels x 3507 pixels
A5: 5.83 inches x 8.26 inches or 1749 pixels x 2478 pixels
If you want to print at a different dpi, then you must multiply the number of this dpi with the inches. For the printing of T-shirts as an example, you would prepare the image at 144 dpi. The ink will bleed on the fabric, finer printing is really not necessary.
To convert centimeters into inches, you have to multiply cm x 0.3937008.