//Instantiating a Workbook object
Workbook workbook = new Workbook();

//Accessing the first worksheet in the Excel file
Worksheet worksheet = workbook.getWorksheets().get(0);
Cells cells = worksheet.getCells();

//Setting the height of all rows in the worksheet to 8
worksheet.getCells().setStandardHeight(8f);

//Setting the height of the second row to 40
cells.setRowHeight(1, 40);

SpreadsheetMLPackage pkg = SpreadsheetMLPackage.createPackage();

WorksheetPart sheet = pkg.createWorksheetPart(new PartName("/sheet1.xml"), "Sheet1", 1);

CTSheetFormatPr format = Context.getsmlObjectFactory().createCTSheetFormatPr();
format.setDefaultRowHeight(5);
format.setCustomHeight(Boolean.TRUE);
sheet.getJaxbElement().setSheetFormatPr(format);

SheetData sheetData = sheet.getJaxbElement().getSheetData();

Row row = Context.getsmlObjectFactory().createRow();

row.setHt(66.0);
row.setCustomHeight(Boolean.TRUE);
row.setR(1L);

Cell cell1 = Context.getsmlObjectFactory().createCell();
cell1.setV("1234");
row.getC().add(cell1);

Cell cell2 = Context.getsmlObjectFactory().createCell();
cell2.setV("56");
row.getC().add(cell2);
sheetData.getRow().add(row);