First things first - how do you choose card? Here a few things that I have found out through making cards.
Card and paper come in a variety sizes, here are a few of the most popular:
A5 148 X 210mm
A4 210 X 297mm
A3 297 X 420mm
Each are double the size of the previous size.
If you fold a piece of A5 card in half it will fit into a C6 size envelope, these are widely available in the High Street, although colours are limited. If you take a piece measuring 87mm off the longest side of A4 you will be left with a square 210mm, fold this in half and it will fit into a DL size envelope.
For other size envelopes you will have to visit various specialist supplies, I have a few mentioned on the links page. There are also lots posted on the forum.
Card and paper should be quoted in either weights or thicknesses. Microns (mic) represents the thickness of the card 1000 Microns equals 1mm thickness. Grams per square metre (gsm) is the weight. I find that the mic is not always quoted and only the gsm available. An ideal weight to make a quality greeting card will be in the region of 220 gsm - 260 gsm, or if the mic available 260 - 350 mic. Thicker than this it may be difficult to fold, and thinner too flimsy. I fold my card by matching the corners and then running something along the spine like my scissor handle or a spoon - something smooth but firm, and I always do this on the back of the card.
If you need to add card onto the front of a card I would choose a lighter weight, it is more flexible, looks neater and does not make the card too heavy at the front. A weight such as 160 gsm, often advertised as photocopying card is ideal. I use glue such as 'Pritt Stick' to attach it, but there are a variety of other methods such as double sided sticky tape or using a Xyron sticker maker.
Paper and card is normally 80 or 90gsm, and a ream is 250 sheets. Paper and envelopes that are described as 'laid' will have a textured stripe.