Size limitations for envelopes, cards and other pieces of mail-under discussion at the U.S. Postal Service for three years-will go into effect July 15.

An experimental local delivery service for businesses also will begin April 30 in three cities - Columbus, Ohio; Gulfport, Miss., and Chicago-providing competition for local delivery companies.

Both actions were approved by the Postal Services board of governors at a meeting in Salt Lake City on Tuesday.

The new size standards, first recommended by the Postal Rate Commission in 1976, effectively ban small envelopes and add surcharges to larger envelopes. Effective July 15, the Postal Service:

Will not accept any letters if they are less than 3 1/2 inches high, 5 inches long or seven-thousandths of an inch thick.

Will assess a 7-cent surcharge for first-class mail weighing an ounce or less and single-piece third-class mail of two ounces or less that exceed any of the following dimensions: 6 1/8 inches high; 11 1/2 inches long, 1.4 inch thick.

Will also add a surcharge for odd-shaped pieces.

The Postal Service said the new restrictions are necessary to reflect higher costs of handling odd pieces, which cannot be put through letter sorting machines.

The local delivery service-which has attracted opposition from private firms in the business-may be expanded to 22 other cities (including Washington) if successful in its pilot months. Packages delivered to selected post offices by 10 a.m. will be delivered by 5 p.m. that day.