A note on sizing and photography
Photographic prints have traditionally been in imperial measurement sizes. Genuine photographic papers with silver halide emulsions still are in imperial measurements.
One inch = 25.4 mm
Most prints today are produced using ink jet printers. This process is quick and far less expensive than photographic processing using chemistry. Unless a printer has specified that they use genuine silver halide papers, the prints will probably be produced using a much cheaper process.
Using a magnifying glass, a digital print will show a mass of coloured dots forming the picture. A photograph produced using silver halide papers and chemistry will show a gradual transition from one colour to another no matter how much the photograph is magnified.
The life of traditional chemistry produced photographic prints is far greater than prints produced using inks.
If a photographer advertises enlargements using A4 or A3 measurements, they will almost certainly be produced using ink. Check how the prints will be produced if you want top quality.
The prices below are current as of Friday 10 June, 2016