Most people find this quite difficult, and get annoyed about not knowing how.
Some people just resize in ms paint then import the sprites to photoshop.
I honestly don't especially like this.
So first, get your sprite sheet.
Next Click on the tool bar where it says,File image ect.
Click edit>Preferances>General.
Then change settings to nearest neighbour.
The rest should be easy.
Just use the free transform tool making your dimensions bigger by 100%'s.
Hope this helps.