ENTER physical SIZE or  pixel SIZE IN BLUE BOXES.    
FILE SIZE   –  PIXEL SIZE  –  SAFE  PRINTABLE SIZE    
ENTER DESIRED PRINT SIZE HERE = VERTICAL
HORIZONTAL INCHES
X    
REQUIRED PIXEL SIZE
Approx.
Maximum Quality PPI X FILE SIZE = MB
Excellent Quality PPI X FILE SIZE = MB
Good Quality PPI X FILE SIZE = MB
Fair Quality PPI X FILE SIZE = MB
ENTER PIXEL DIMENSIONS HERE = X PIXELS    
 Printable SIZE at 300 dpi (Maximum)  X INCHES
 Printable SIZE at 200 dpi (Excellent) X INCHES
 Printable SIZE at 150 dpi  (Good) X INCHES
 Printable SIZE at 100 dpi  (Fair) X INCHES
iPhone Camera Specs.
IPhone 4,5,6 = 8 Megapixel Camera- photo size is: = 2448 x 3264 ppi             approx. 8 x 11 print size at maximum quality, 16 x 22 at good quality  
IPhone 6s, 7, 8, X .12 =  12 Megapixel Camera- photo size is = 3000 x 4000 ppi approx.  10 x 14 print size at maximum quality, 20 x 28 at good quality  
How to do the calculation: The first step in calculating what size print you can make is to find the image’s pixel dimensions and jot them down.    
 MAC: In Photos, select a picture and press Command-I to open the Info panel. The dimensions appear in the second section of the panel.    
PC: Type "File Explorer" into the search bar.  Open File Explorer.  Find the image you want to check.Place your cursor over the image name until the hover text with the image dimensions appears.    
   
* There’s a FREE downloadable iPhone app called “Photo Inspector” that will display the pixel data, file size, etc. for any picture on your iPhone.