You can and ill show you with your G-Tar:
This is how it looks in its Gravatar state:
But you can make is Bigger:
This is normal Size:
This is what would do =0:
All you have to do is at the end of the Image Link type this(=) then pick a number. Any number above 100 will make it bigger. Any number under 100 will make is smaller. If you dont put the = sign then it will stay the original size.If you set it equal to Zero it would stay the same.