This may happen because the resolution and pixel dimensions of your image file are not high enough.

Each product has a maximum orderable size which can be found on each product page:

This information can also be found in the configurator under "Image size & crop photo":

For the same product, the maximum orderable size in the configurator may be different than the one previously mentioned on the product's page:

In this case, the reason may be that the file uploaded on the website has a low resolution. To avoid pixelated prints, the website only presents sizes that will correspond to the image you wish to print.

Please note that the aspect ratio of your file may also influence the available sizes presented in the configurator.