Prompted by a retailer who discussed a "large" screen size format, we looked at what browser sizes were accessing a typical "retail" e-commerce website.  The site had about 200,000 visitors in the recent month that we looked at in Google Analytics.

The data is summarised in the table below, with 92% of all visitor sessions coming from the 20 most common screen sizes listed below (width being shown first).

It was a relatively straightforward task to comment upon what sizes represented what kinds of devices (assisted by a pivot table with a dimension showing operating system):

Our conclusions:

• Design for 768 * 1024 (iPads in portrait mode), accounting for nearly a quarter of all traffic
• 768 * 1024 represents a good balance point between using all the available screen size and not having horizontal scroll bars on fixed zoom browsers (desktops & laptops) while still allowing the customer to see detail in the content on the site without having to zoom on a dynamic zoom device (tablets)
• It’s more important to review how customers are using devices on your sites than what devices they’re using (iPad in portfolio mode)
• Don't worry about gaps at the edges of high resolution browsers (e.g. large iMacs), which don’t actually account for a lot of the traffic
• Building the website with "responsive" design would still need to account for the different screen size "breakpoints" for determining which display size is used