We recommend a minimum 40cm x 40cm for canvases with detail, smaller canvases should be used for test trials or canvases without sharp lines, faces etc.
Based on our years of experience crafting diamond paintings, the bigger a canvas with a full pasting area the better. The majority of our canvases are full pasting area's however - we do have some that are not, which are clearly described when they are not full pasting area's.
The bigger the picture correlates to the better image quality and resolution, effecting the image sharpness and details that can visibly be seen on the canvas.
If the canvas doesn't have an expectation for a realistic representation of a subject, or images without defined faced or lines, a 40cm x 30cm should be sufficient without you sacrificing too much of the pictures quality detail.
40cm x 30cm can be sufficient for a single portrait of a face, however if there's multiple faces, you may want to consider 60cm x 50cm and above, to have a detailed enough representation of the faces etc.