Make photos smaller before printing them ?

Ok so the wife is 50 years old soon and i have decided to do her a photo album of all the years gone by. The family are providing me with loads of pictures but as you can imagine they are of various sizes and the photo album only holds 6 x 4 pictures

i will scan the pictures from printer to pc

Whats the best way to shrink these down before ordering them online without losing quality ?