Since Portfolio is a responsive website editor, there are a number of things to consider when it comes to "size."
It all starts with your Theme. While there are many themes, the biggest distinction between them is the placement of the Navigation on your site. Sites that have a Vertical Nav (aka, a column-style Nav on the left or right side of your site) will have less viewable space for images than the sites that have a Horizontal Nav (aka, a Nav that lies along the top of your site).
Website Container vs Page Container
Next, after you've chosen the Theme, it's important to determine if you want to restrict how wide a your site can get or not.
For example, if you set a max width of 1400px to your site, site content (Pagae Covers, Images, Text, and Vertical Nav, if applicable) will fill the browser window up to 1400px. After that, the browser adds spacing to the left and right of the site content.
Alternatively, you can remove any max width set on your "Website Container". If you set the content % to 100, this will allow the page to fit virtually any browser size.
You can access the "Website Container" Menu from the Gallery Editor Remote:
You can ALSO set max widths for your Pages. This would be useful if you want your Gallery pages to view at 1600px but you want your Pages to view at 1400px.
We recommend deciding your width from the Website Container rather than the Page Container. The Website Container overrules any max widths set within Page Containers that are greater than the Website Container. For example, if you set a Page Container's max width to be 1400px, but your Website Container is set to 1200px, your pages will max out at 1200px.
Best practice: If you want the largest possible images, don't set a max width on either the Website Container or Page Container. Depending on the Theme, this may result in very large Cover Images in your gallery. To change the size of the Cover Images, click Cover Images in the Gallery Editor Remote and then adjust the number of columns or increase/decrease the gutter and margins.