With Royal Mail - for inland (UK) the definitions are this:
Letter : Max length: 24cm; Max width: 16.5cm; Max thickness: 0.5cm; Max weight: 100g
Large Letter: Max length: 35.3cm; Max width: 25cm; Max thickness: 2.5cm; Max weight: 750g
Beyond those you're into small/medium/large parcels
For international:
Letter: Max length: 24cm; Max width: 16.5cm; Max thickness: 0.5cm; Max weight: 100g; Minimum size: One surface must be at least 9cm x 14cm
Large Letter: Max length: 35.3cm; Max width: 25cm; Max thickness: 2.5cm; Max weight: 750g
Small Packet: Minimum size: One surface must be at least 9cm x 14cm; Maximum size: The combined length + width + depth must be no more than 90cm with no single side longer than 60cm; Maximum weight: 2kg
Tubes: Minimum size: Length plus 2 x diameter should be at least 17cm, with the length no shorter than 10cm; Maximum size: Length plus 2 x diameter should be no greater than 104cm, with the length no greater than 90cm; Maximum weight: 2kg
Beyond that you're into kidney sales territory and international parcels... if it's possible to split your >2kg delivery into two parts it's usually cheaper to send 2 small parcels than one large one...
Oh, and all of this is at the mercy of the person on the counter at the Post Office - who has an epic volume of things you're not allowed to post at all.... one of which is "items manufactured from cellulose-based pulp" (you know, like paper...)
hunnymonster, do you still have any sets available? I would buy a set off you
I'll send you a PM.
I do