When you’re sending your PDF portfolio out to publishers and agents, the Writers’ and Artists’ Yearbook (UK) recommends about 20 images, the Children’s Writers’ and Artists’ Yearbook says 10 to 15 is a good number.
But what about the size of the PDF, if you’re sending it as an email attachment?
Anything under 5mb should be fine.
As far as I can tell, the most common screen resolution is 16:9 ratio (for example 1920×1080 resolution). The problem is that gives you a long, thin image, and not many of my pictures fit into that. I ended up going for 2,000 x 1,400 pixel layout, which is closer to regular A4 landscape dimensions.