The most common order options are 6″ x 4″ and 8.5″ x 5.5″ on 14 Pt. Gloss Cover stock. Almost always, Postcards are printed in full color on both sides (4/4) or full color on the front and black on back (4/1) and coated UV 1 Side.
14 Pt. Gloss Cover stock. Postcards are printed in full color on both sides (4/4) and coated UV 2 Sides.
16 Pt C2S Gloss Cover
You may be familiar with this type of stock from heavy bookmarks or other products printed on a semi-rigid card stock.