To find out the size of an image in pixels. do the following
1- Download the image if not on your PC
2- Right click on the image
3- Click properties
4- click details
Under the image section you can read the dimensions of the image ( width and height) in pixels.
That is it.
In order the change the size of the image you need to load it to photo editing program such as paint, Photoshop. or you can an online editor such as pixlr.com