If you use a picture frame as big as A4 paper, a 12 inch frame is enough.
Because the size of A4 paper is: 2 10*297MM, and 1 inch is about 3.33cm..A4 paper is defined by ISO2 16, and its specification is 21* 29.7 cm (210 mm × 297
Other dimensions: 1 inch: 1* 1.5 (inches) to 2.54*3.7 (centimeters), 2 inches: 1.5*2 to 3.75*5.3, 3 inches: 3*3.6 to 5.
Generally, you choose according to the size of your photos. A few inches of photo frame can hold several inches of photos. For example, a 7-inch photo frame can hold photos of 12.7* 17.8cm, and an 8-inch photo frame is suitable for photos of15.2 * 20.3cm.. You can also use a smaller photo, and the periphery can be inlaid with cardboard.
Note on photo wall photo frame: put photos or pictures into a blank photo frame and prepare a photo frame with pictures or photos. Fix the drawing template on the wall with I-nails. After the position is fixed, use transparent glue to avoid displacement. Note that the drawings must be horizontal and vertical, and cannot be tilted. Try to flatten the drawings and stick them on the wall, and there can be no wrinkles and bulges. Nail the plastic hook on the wall according to the circle above the template, and don't let the drawing loose or shift when hammering the hook.