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
H&M Home
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