What’s smaller than a Sears store but still larger than the average supermarket? Sears Roebuck and Co., Hoffman Estates, Ill., plans to convert some of those stores it bought from Kmart and Wal-Mart last year into a new concept this spring. The new Sears Essentials stores will offer Sears' standard departments, such as apparel, appliances, lawn and garden and electronics, but will add groceries.The mix may look like that of the Sears Grand stores created last year but at half the size or less. Averaging about 84,000 sq. ft., they'll be nearly twice the size of a grocery. The first 25 such stores will open this spring, possibly in Michigan, California, Florida and Illinois.