I'm assuming you are using a windows operating system.
When you have scanned your photo, make sure it is saved as a 'jpeg' or 'jpg' file. Some scanners have different files such as 'TIFF' as their default setting but you root about you'll find in the scanning menu there is an option for save as - thgen a list of file types.
When you have saved the .jpg file, go out of the scanner programme and open the file by right-clicking on it and choosing 'open with > microsoft office picture manager'.
In this programme, at top right of screen you will see 'edit' - click that - a menu opens up on the right of screen - choose resize.
Then you can choose the size you want, save and you're done.