In the Joomla backend, go to the Plugin Manager to change the ImageSizer default settings. In the Basic options panel, set Min Size X(Thumb) and Min Size Y (Thumb) to 450. Don’t let the label Min Size confuse you: this is not about the minimum image dimensions, it’s about the maximum dimensions of the “miniature” or “thumbnail” image. In other words, the maximum horizontal and vertical “thumbnail” dimensions will never be wider or higher than 450 pixels. Of course, you should choose another value here if that fits your site design.