How big is a large letter royal mail

According to Royal Mail's guidelines, a Large Letter is defined as a package that measures up to 24 cm (9.4 in) in length, 17 cm (6.7 in) in width, and 3 cm (1.2 in) in depth.

In terms of weight, a Large Letter can weigh up to 750g (26.4 oz).

Here's a summary:

Keep in mind that these dimensions and weight limits are subject to change, so it's always a good idea to check with Royal Mail for the most up-to-date information.