Picking the wrong paper size is one of the most common (and most avoidable) printing mistakes. Here's a quick reference, with the real numbers behind each size, not just the names.
A4 — the everyday default
A4 (210 x 297mm) is the standard size for almost all everyday printing in India — documents, resumes, forms, assignments. If you're not sure what size you need, A4 is very likely the right answer.
Letter — common in US-formatted documents
Letter size measures 216 x 279mm (8.5 x 11 inches) — slightly wider and shorter than A4. If a document was originally created in the US (some templates, forms, or files from American companies), it may default to Letter size — printing it on A4 can sometimes cut off the bottom margin slightly.
Legal — for longer forms
Legal size is 216 x 356mm (8.5 x 14 inches) — longer than both A4 and Letter, and mostly used for certain legal or government documents that need the extra length for fine print. It's uncommon for everyday printing.
A3 — for posters, charts, and large layouts
A3 (297 x 420mm) is exactly double the area of A4. Useful for posters, large diagrams, or anything where readability at a distance matters. Not every printer supports A3, so it's worth confirming with the shop first.
Why A-series sizes always relate cleanly, and Letter/Legal don't
A4, A3, and the rest of the "A series" all follow the international ISO 216 standard, built on a fixed 1:√2 ratio — fold any A-series sheet exactly in half and you get the next size down with the identical proportions (A3 folded in half is A4, A4 folded in half is A5, and so on). Letter and Legal, used mainly in North America, don't follow that same single ratio between length and width, which is the real, mathematical reason scaling a US-formatted document cleanly onto A4 (or the reverse) doesn't always come out perfectly proportioned.
Quick tip
When in doubt, check the document itself — most PDF and Word files show their intended page size in the print settings before you even reach the shop.
A5 and other sizes you'll see less often
A5 (148 x 210mm — exactly half of A4) shows up for smaller booklets, flyers, and some invitation cards; it's the same A-series math as A3/A4, just one step smaller. Tabloid (11 x 17 inches, the North American equivalent role to A3) covers a similar "bigger than standard" niche in US-formatted documents. None of these need memorizing — the practical rule stays the same: A4 covers the overwhelming majority of everyday printing, and everything else is a specific-purpose exception worth confirming with the shop rather than assuming.
Orientation matters as much as size
Portrait vs. landscape is a separate choice from paper size, and mixing them up is one of the most common avoidable mistakes — a wide table or chart designed for landscape will print awkwardly cramped, or get cut off at the margins, if sent through as portrait by default. Checking both size and orientation together, not just one or the other, is the real habit worth building before sending a job to print.