Specify size when exporting using Photoshop generate images

what is the latest version of photoshop

Yes, I agree with you. Centimeters must not be the criteria, the developers could add or generate new paramenter. I am also not in the favor of Pixels. But today's era of Artificial intelligence could help us better. 

It sounds like you're trying to use Photoshop's naming structure to specify export sizes for your image assets, but it's not working as expected. According to the guide, using a name like "300mm x 20cm Rounded_rectangle_3.png" should let you specify the export size directly in the layer name. However, you've found that this doesn't work and the assets are being cropped to their minimum size with no extra space.

First, ensure that you're following the correct syntax for naming. Photoshop typically uses pixels for dimensions in the naming convention, such as "300x200 Rounded_rectangle_3.png" (without units like mm or cm). Double-check the guide to confirm if it supports measurements in mm or cm. If it doesn’t, convert your desired dimensions to pixels.

Additionally, there might be a setting or preference in Photoshop that needs to be adjusted. Check the "Export As" settings under File > Export > Export As to see if there's an option that influences how layer sizes are handled during export.

If adjusting these settings doesn't help, your current workaround of creating a mask with the correct size in each layer is a good approach. While it's more time-consuming, it ensures that your assets export with the desired dimensions.

You might also consider updating Photoshop if you're not using the latest version, as there could be a bug or limitation in the version you're using (Photoshop 2022 on macOS Big Sur 11.2.2). Updates often include fixes and improvements that might resolve your issue.

Hopefully, this helps you resolve the issue. Good luck!

Actually "centimeters" is a meaningless concept in Export and I can't imagine how that could work.

Export sizes the image in pixels only. Measurement in cm would be determined by the ppi (pixels per inch) number, but there is no ppi in Export. If there already is a ppi number in the file, it's stripped out.

It's important to understand that Export is for web/screen/mobile, and ppi (and hence physical size) simply doesn't apply on screen. Ppi is a print parameter. On screen, the "resolution" is the native screen resolution of whatever device you're viewing the file on.

Do these links help?

https://helpx.adobe.com/photoshop/using/generate-assets-layers.html

https://github.com/adobe-photoshop/generator-assets/wiki/Generate-Image-Assets-Functional-Spec