* Value of 26 pct stake estimated at 1.5 bln-1.6 bln euros (Adds details on transaction, context): FRANKFURT (Reuters) - German regional utility EWE [LANDWE.UL] will offer investors a minority stake next month, three people familiar with the matter said, in a deal that could value the whole group at up to 6.2 billion euros ($7.2 billion). After advertising the offer of the 26 percent stake, probably in late February, prospective buyers would have four weeks to indicate their interest, one of the people said.