Hmmm. Well, in paper crafting you're always going to have some scraps left over; it's the nature of the beast. ;)
For a standard A-2 card (measuring 4 1/4 x 5 1/2"), you cut a letter size sheet of card stock (8 1/2 x 11") in half so you have two 8 1/2 x 5 1/2" pieces.
Then score the 8 1/2" to 4 1/4" and you can fold the piece in half, yielding 2 A-2 cards from one sheet of card stock. This size card goes 1st class in the mail and is a standard in the US.
Many people generously include the source of supplies/materials used in their projects, even right down to an exact color name, but unless the project required very specific measurements (such precision folding, etc.), they more than likely didn't record or document that info.
So, unless someone specifically lists the measurements of each piece/layer on their card, you will have to make an educated guess, when copying (also affectionately referred to as CASE-ing, in the stamping world--Copy And Steal Everything).
Most stampers become pretty adept, as they learn, to eyeball things. ;)
Happy stamping to you, and I'm sure if you have questions about a specific project you find in the gallery, you can always private message a polite inquiry to the artist for more info.