In SimpleumSafe for iOS, you can view files in list view or thumbnail view.
In list view, the thumbnails are currently a fixed size.

In thumbnail view, you can apply the two finger zoom gesture to resize the thumbnail. Place two fingers on the screen near each other. Spread them apart to zoom in, or move them toward each other to zoom out. The size of the thumbnails then increases or decreases and you can adjust everything to suit your needs