Prints should be sized to fit either the entire frame (without mat), or the mat's opening. Frame packaging/descriptions at retailers will indicate which print sizes it can hold.
IKEA: HOVSTA frame
IKEA: RIBBA frame
Target: Matted PS Narrow Rounded Gallery Frame
West Elm: Wood Gallery Frames Wheat
Target: Elena Photo Frame - Natural Wood - fits A3 print with mount, or A2 print without mount
This adorable set of 6 baby animals you might find on a safari in Africa or Asia is available as printable art or prints shipped to your home. Pictured are 16"x20" prints in same-sized frames - the tiger wallpaper is from anewall.com. Included animals: elephant, zebra, monkey, tiger, giraffe and lion.