Edit: Below solution works fine only with DynamicJasper library rather than DynamicReports. There must be similar operations in DR API as well i believe (not sure though - my exposure with DR is only very limitted :().
A combination of DynamicReportBuilder::setHeaderHeight(..), setDetailHeight(..) and JRHtmlExporterParameter.IS_USING_IMAGES_TO_ALIGN parameter setting must do it?
// build dynamicreport instance
DynamicReportBuilder dynamicReportBuilder = new DynamicReportBuilder();
dynamicReportBuilder.setHeaderHeight(50); //sets the column header height
dynamicReportBuilder.setDetailHeight(50); //sets the detail band rows height
DynamicReport dynamicReport = dynamicReportBuilder.build();
// build jasperprint..
JasperPrint jasperPrint = DynamicJasperHelper.generateJasperPrint(
dynamicReport, new ClassicLayoutManager(), dataSource, new HashMap<String, Object>());
// export as html
JRExporter exporter = new JRHtmlExporter();
// tell jasper not to use images for aligning
exporter.setParameter(JRHtmlExporterParameter.IS_USING_IMAGES_TO_ALIGN, Boolean.FALSE);
exporter.setParameter(JRExporterParameter.JASPER_PRINT, jasperPrint);
exporter.setParameter(JRExporterParameter.OUTPUT_FILE_NAME, targetFile);
exporter.exportReport();