You can easily create as many sizes you need to a particular image.
Click on the image you would like to change the image size.
Click on the resize tab.
Use either the slider to get the exact size you would like, or type in the dimensions you need.
Click Save new size
You have now created a new version of your original image that you can find in the drop down list on your top right.
You can delete a size version by choosing the size you want to delete and click delete size.
Note. If you delete a size version, the original file will always be stored.