function cropSheet() {
  const dataRange = SpreadsheetApp.getActiveSheet().getDataRange();
  const sheet = dataRange.getSheet();

  let numRows = dataRange.getNumRows();
  let numColumns = dataRange.getNumColumns();

  const maxRows = sheet.getMaxRows();
  const maxColumns = sheet.getMaxColumns();

  const numFrozenRows = sheet.getFrozenRows();
  const numFrozenColumns = sheet.getFrozenColumns();

  // If last data row is less than maximium row, then deletes rows after the last data row.
  if (numRows < maxRows) {
    numRows = Math.max(numRows, numFrozenRows + 1); // Don't crop empty frozen rows.
    sheet.deleteRows(numRows + 1, maxRows - numRows);
  }

  // If last data column is less than maximium column, then deletes columns after the last data column.
  if (numColumns < maxColumns) {
    numColumns = Math.max(numColumns, numFrozenColumns + 1); // Don't crop empty frozen columns.
    sheet.deleteColumns(numColumns + 1, maxColumns - numColumns);
  }
}
Trims all of the unused rows and columns outside of selected data range.

Called from menu option.