normal letter size

Ideal for mailing greeting cards, regular letters and postcards, the maximum size that would fall under a ‘letter’ is 165 mm x 240 mm. We would recommend using an envelope no larger than C5 to fit these restrictions comfortably and avoid any excess costs. The envelope and contents must also weigh under 100g.
For mailing contents such as A4 sized documents, DVDs, certificates, large greeting cards and magazines you would need to send as a large letter. The maximum size is 250 mm x 353 mm and and a maximum weight of 750g so a C4 envelope is a good choice.

For custom orders, call 1-877-683-5673
Our size guide will help you select the perfect envelopes to fit your needs. Can’t find what you’re looking for? We can custom make it for you!

A4 sheets have a width of 210 mm and a length of 297 mm. A4 paper size sheets are part of the ISO 216 system that has been internationally adopted. The size is printable in most common home and office printers and has become the standard copy paper sheet.
The image above showcases the two sheet sizes side-by-side in both millimetres and inches. The two are very similar in visually, though you’ll notice that the US Letter is wider whereas A4 sheets are longer.

References:

http://www.envelopes.com/envelope-sizes
http://uk.onlinelabels.com/articles/difference-between-us-letter-a4-paper-sheets
http://rasmussen.libanswers.com/faq/33178