Pixels - most resizing programs will have that option. If you choose a bigger image it will display them at 100 x 100 pixels, so will distort them if they're not already square (and will still be as big in terms of file size).
Let me know if you need any help - I can always resize something for you if you like.
I made him blink Al. Hope you don;t mind. just makes him extra cute. Am making some little pig toons for each section of my website inspired by my gorgeous Hugo you made me. Wish I could draw like you Al