It really depends on your preference:
Cold press paper has a slightly textured surface. It is the paper used most often by watercolour artists because it's good for both not only large areas of wash but also for a good amount of detail. Also, the textured paper allows to shape things easier!
Hot press paper has a fine-grained, smooth surface, with almost no tooth. It's ideal for large, even washes of one or two colors, drawing with pens and ink wash, and for painting fine details. The smooth surface makes it easier to draw and shade with dry mediums.
What are the measurements of the postcard and the box?
A box weighs about 500g (roughly 1 pound) and contains 100 postcards. Each postcard measures 14.8 x 10.5 cm (5.8 x 4.1 in). Just the right size to stamp and mail right away!