to define a size using an default size : {define_derivative name='NAME' type='TYPE'} where NAME is the name of the derivative created (use it like $pwg→derivative($NAME, $src-image), and TYPE can have the values : square, thumb, 2small, xsmall, small, medium, large, xlarge, xxlarge. The sizes corresponding to these types are available in Configuration → Options → Photo sizes
to define a new size : {define_derivative name='NAME' width='WIDTH' height='HEIGHT' crop=true/false min_height='MIN_HEIGHT' min_width='MIN_WIDTH'} where NAME is the name of the derivative created (use it like $pwg→derivative($NAME, $src-image), WIDTH and HEIGHT are required and must be numeric values, the attribute crop is optional (default value false), and the attributes min_width and min_height are also optional (only numeric value)
to get a url of a picture according to the NAME derivative : {$pwg→derivative_url($NAME, 'src of the original picture')} OR {assign var=my_derivative value=$pwg→derivative($NAME, 'src of the original picture')} with {$my_derivative→get_url()}