If you want a designer create a custom postcard for you you need to provide these information.
Standard size: 5.45”x4.21”
4″ x 6″
5″ x 7″
If you want to use specific image or logo of your company, then you need to provide these images as large printable quality file.
A photo resolution should be 300 dpi
Then provide the name of the font for your postcard text. You can choose any of these fonts online: https://www.google.com/fonts
Full Bleed Size (starting document size)
6.12″ x 4.12″
155 x 105 mm
1835 x 1235 pixels
* Finished artwork should use the full bleed dimensions for best results.
Document Trim Size (final size after being cut)
6.00″ x 4.00″
152 x 102 mm
1800 x 1200 pixels
Resolution: 300 DPI
* Please be sure to keep all text within the safe margin.