The standard size of typing paper adopted by the International Standards Organization. It measures 210 mm wide and 297 mm long (about 8 1/4 x 11 3/4 inches). It is used in most countries of the world, except the US and some neighboring countries where letter-size paper (8 1/2 x 11 inch) is used. See also paper sizes.
Successive paper sizes in the series A1, A2, A3, and so forth, are defined by halving the preceding paper size across the larger dimension. … The most frequently used paper size is A4 measuring 210 by 297 millimetres (8.27 in × 11.7 in).
A quire of paper is a measure of paper quantity. The usual meaning is 25 sheets of the same size and quality: 1⁄20 of a ream of 500 sheets. Quires of 25 sheets are often used for machine-made paper, while quires of 24 sheets are often used for handmade or specialized paper of 480-sheet reams.
GSM is an acronym standing for ‘Grams per Square Meter’. Quite simply, it allows print buyers and print suppliers to know exactly about the quality of paper that is being ordered. The higher the GSM number, the heavier the paper. If you’re buying print then make sure you know what you’re talking about. Being specific about the GSM you’re after can make all the difference and you’re likely to be taken more seriously by a printer if you ask for a specific GSM rather than just asking for thick or thin paper.