The simplest approach would be to query the ADMIN_GET_TAB_INFO
table function:
SELECT
data_object_p_size +
index_object_p_size +
long_object_p_size +
lob_object_p_size +
xml_object_p_size
FROM TABLE (SYSPROC.ADMIN_GET_TAB_INFO('YOURSCHEMA', 'YOURTABLE'))