I assume that in IrfanView you chose Print and then selected Custom to enter your 5x7.
If you are using a normally setup UK computer that will be 5 x 7 centimetres, which is of course 2 x 2.8 inches.
You don't say what size paper you are using. If you are actually using 5 x 7 paper, first click the printer setup button and choose the correct size and orientation of paper. Next, select Best fir to page (aspect ratio).
If you are using another paper size, select custom, make sure that Aspect Ratio is checked and type in either the width or height in centimetres. You can see the effect this will have by looking at the preview which is built in to the print screen. Also if you look at the bottom right you will see the selected size shown in both centimetres and inches.