The most common size is one-quarter inch larger than the piece of card stock for which you are making a layer or mat.
If that sounds large, it really isn't.
The layer, or mat, is a quarter inch larger in height and width, but only gives you an 1/8" on each of the four sides.
Of course, you can make larger or smaller layers for the desired look, but you will find 1/4" larger is the most common size you will use for a card making layer.
There are times a smaller matting layer is desired to provide just a hint of an accent color.
As well as, there are times a larger layer is attractive.
A word of caution when you choose to make a smaller matting layer, such as 1/4" larger in height and width, that only gives you an 1/8" on each of the four sides.
This is when you need to take extra care in measuring and cutting accurately.
That is because with such a small variance in sizes between the layers, it is very obvious when you are off; it will make your handmade greeting card less attractive.
Let me give you another hint when making matting layers for your handmade cards: measure your first card stock layer to the closest 1/4" increment.
It is then easy to make your next layer because we quickly recognize measurements of 1/4", 1/2", 3/4" and a full inch.
There will be times when the perfect size layer is with an 1/8" increment because 1/4" simply does not either look right for one reason or another.
Now you will need to learn how to read 1/8", 3/8", 5/8", 7/8".
The more you do, the easier it will become.
If you go any smaller to 1/16", you will need to adjust accordingly.
To make card making easy, do your best to cut your layers in 1/4" increments.
You will find most common size layer you will make will be 1/4" larger.
Too many times I have watched card makers take a piece of card stock to the cutter and just trim it without regard to the size to which they are cutting it.
Then when they wish to make a layer for it, they "eyeball" instead of measuring when cutting the layer.
This virtually always makes for a lopsided mat and is frankly unattractive.
Making layers for your handmade greeting cards is easiest when you try to keep the layers to measurements of 1/4".