Media Capacity is the maximum amount of media that a printer can hold at any one time. Given that most printers use a roll fed paper roll then the size of the roll and the size of the printer dictate this.
The larger the printer and the media roll the larger the capacity of the printer and the more prints you can print without having to change the media.
Usually this isn’t an issue unless the printer is being used in a photo booth or in an unattended operation as changing the media is a relatively quick and simple operation. Print Capacity is usually shows in the number of 6×4 prints that a printer can hold when the paper roll is full. This figure is halved if the user is using 8×6 media as these prints are twice the size. A printer with a media capacity of 400 6×4 prints will be able to print 200 8×6 prints.
Need more capacity?
As mentioned above Media Capacity is only really an issue where the printer is unattended like a photo booth. The simple way to increase media capacity is to use two printers connected to one PC or use something like the Mitsubishi D707DW which is basically two printers in one and has the ability to print two different print sizes e.g 7×5 and 8×6.
For most people media capacity isn’t really a major reason to choose one printer over another.