#10 envelopes are the “normal” size and weight for mailing, measuring 4 1/8 by 9 1/2 inches and weighing 0.0208 ounces.
You won’t find an address window on a standard envelope because the front is always closed. This envelope style is the standard for both professional and personal correspondence. The amount of postage needed can vary depending on factors including the weight of the contents and the envelope. Standard-sized, letter-weight mail weighing less than 13 ounces can be dropped off at any USPS drop box; packages weighing more than that must be dispatched from a shipper’s location.