Hi,
Most Post Offices will have a template for you to try your letters against. Or try here.
http://www.postoffice.co.uk/portal/po/content1 ?catId=19100177&mediaId=19100217#1400002
How large can it be?
Mail is now priced depending on size as well as weight, with three distinct size formats: Letter, Large Letter & Packet.
The maximum dimensions for a Letter are 240mm x 165mm and no thicker than 5mm. It must be no heavier than 100g.
The maximum dimensions for a Large Letter are: 353mm x 250mm and no thicker than 25mm. It must be no heavier than 750g.
Any item larger or heavier than these is classed as a Packet, with the maximum dimensions being 610mm x 460mm x 460mm.
There is no wieght limit for 1st Class items when sent as a Packet. The maximum weight limit for a 2nd class item is 750g. If a 2nd class item is sent as a Packet the maximum weight limit then becomes 1000g
Cylindrical packets, such as cardboard tubes, have a maximum length of 900mm. However, the length of the tube, plus twice the diameter must also not exceed 1040mm.
This story ran on Breakfast News last week and the example letter the reporter was holding was huge (a bit bigger than A4 size).
Unless you send really big cards through the post normal first and second class rates should do. Last dates are 17th Dec for second and 21st. Dec for first mainland UK and Channel Islands.