Also would have been posible by just coding it smaller not resizing. I'll leave instructions anyway in case other people are wondering..
The gif in my sig:
But the size I have it is
<img src="http://26.media.tumblr.com/tumblr_lpi7pd4dMg1qhfqbko1_500.gif" width=250px />
You'd just need to change the URL to the link to your image and then change the width to whatever you want. You can also use height=200px (with whatever size you want, of course.) It's not necessary to specify both width and height, but you can if you want to. But you might end up with
images not getting scaled/resized properly.
It's basically the HTML version, not the BBCode (using [img] tags) because resizing in that doesn't seem to work here.