There are 2 ways to do this, both a total pain.
1) You can build the ps yourself using {block:Photos} and {JSPhotosetLayout} and javascript. not fun to do because of the differing heights of images in multi-columned rows so you have to make them as background images and then find the right height ect. ugh but it is possible, I've done it.
2) Still using js or jquery you can grab the iframe and replace it with another iframe with the '/500' at the end of the src which denotes the width with a different number. Problem is that you need to specify a height of the iframe which can be roughly calculated by the old height * (new width / old width), but since they have a fixed photo margin of 10px they don't scale rationally. In general if you're trying to make it bigger minus a few pixels from that rough height and vice versa for making a smaller ps.
Feel free to try these methods but if anybody copies my code directly be warned I will cut you. (not joking, it took forever to figure these out and I don't want it nicked like my like button sigh)