Forms often give the photo size in centimetres, like 3.5 × 4.5 cm, instead of pixels. Type the size and DPI and the photo is resized and cropped to fit exactly.
At 200 DPI it is 276 × 354 pixels; at 300 DPI it is 413 × 531 pixels.
Use the DPI your form mentions. If it does not say, 200 DPI is common for exam forms and 300 DPI for printing.
No.