Do you want to generate thumbnails or restrict the size with HTML?

If you want to use HTML, you can use the CSS statements "width" and "max-width". The latter is CSS 2.0 and isn't supported by older browsers.

<img src="..." style="width: 350px; max-width: 300px" />

You have to add these information to your template.

If you want to generate thumbnails I would recommend GD::Image::Thumbnail and Image::Magick::Thumbnail