Do you have photoshop? If so open image, click on 'Image' menu at the top of the page and then 'image size' from the drop down menu. It will let you resize in pixels or percent.
MS Paint also resizes. Open the image click on 'Image' menu at the top of the page, then choose 'stretch/skew' from the drop down menu. You resize in percent. If you put 50% in both 'Stretch' boxes it will halve the size, if you up the values (110%) it will make the image bigger. Sorry if this doesn't make much sense, I'm not very good at explaining things like this.
Hope it helps though!