A rule of thumb when creating any design for printing is to leave space for bleed. A design lacking space for bleed may get cut off, leaving out necessary content.
Content should be away from the edges, as it can cut off from the edges. You can also set a bleed line to mitigate or avoid this, which can be easily done from the editor. A bleed line of 2 mm is sufficient in most cases.
To add your bleed line, open your design in the editor, and browse the right contextual menu.
Under the 'Layout' section, click the 'Bleed' toggle. Set the bleed value of your choice. Select the 'Unit' dropdown to change the value to either millimeters or inches. Save and download your design via the Order History page.
Selecting print dimensions
If setting bleed does not fix the issue, you can also set custom dimensions for your order from the download page.
In the download page, click 'More Options', and select the page dimensions that best work with your printer. You can also change the way how resize interacts with your design by allowing your design to either stretch, crop or fit on the canvas when resized.
Once you've made the change, click 'Download' and you will get the resized design.