Actually it can be done with the filter:
const { createHigherOrderComponent } = wp.compose;
const withCustomClassName = createHigherOrderComponent( ( BlockListBlock ) => {
return ( props ) => {
if(props.attributes.size) {
return <BlockListBlock { ...props } className={ "block-" + props.attributes.size } />;
} else {
return <BlockListBlock {...props} />
}
};
}, 'withClientIdClassName' );
wp.hooks.addFilter( 'editor.BlockListBlock', 'my-plugin/with-client-id-class-name', withCustomClassName );