"...give to my sister, they are too big"
How though, does she view them - what OS and what program.
The problem, as always with pictures, is twofold - how many pixels do they have (and how that relates to the screen) - and are they zoomed in the picture viewer.
Some picture viewers automatically zoom in and out, so that the picture, whatever the number of pixels in it, is viewed at a sensible size - Others don't so the picture appears on the screen at it's actual pixel size, and usually too large, sometimes too small.
Check the actual picture size before you give or send them to her, and resize them to say, 400 pixels wide. Then assuming. they are viewed on an 800x600 screen, they will be half the size of the screen.
Or suggest she gets a viewer that does automatic zooming.