1) What is the best image size for Slider?

An optimal dimensions for your slideshow images need to be 940px wide by 442px high. If you are using  video, Just grab embed code that will give your video a width of 940px.

2) What is the best image size for  Home page top promo box?

First 2 images can be: 300px × 324px And another 2 small images cane be 300px × 150px

3) Is there a standard image size for products?

Optimal image size can be at-least 800px x 950px minimum ,but you can use more than this if you have.

4) What is an optimal size for Category/Collection banner?

Category Banner can be : 1140px × 340px