Is A4 paper considered a standard photo size?

A4 paper (8.27x11.69 inches) is not a standard photo sizes but is commonly used with enlargement prints or collage prints. Visuals Clipping suggests trimming A4 prints when you are going to frame them using standard size frames. Although it is not a photo standard, A4 is convenient in personal work and home printing systems.

Read more, https://www.visualsclipping.com/blog/standard-photo-sizes/