Resizing an image

Hi:

I have a set of SVG files that I want to rasterise. The thing is, each one of them has a circle and I want that circle to be the same size in every image.

Is there any easy way to do this with Illustrator/Photoshop?

I guess there must be a tool to select part of the image and resize it according to that, but I haven’t found it.

Thanks in advance.