How do you photograph or scan a store brought sewing pattern ?

l,

As I (mis)understand it:

At first forget about the exact size, and start with a definite oversize.

0) On the paper with the sewing pattern, add a vertical/horizontal line with an accurate defining length, it could be 100 mm or 4" = 288 pt or whatever, in the unit used;

1) Either scan at highest possible resolution and size if possible, or take a high resolution photo straight on (as Ton has said already) in clear daylight or with a light source that gives an even background.

2) Open a new AI document with the desired Color Mode and unit;

3) Place the scan/photo as a linked file at full size;

4) Zoom in on the defining length line so it fills the creen, then ShiftClickDrag with the Line (Segment) Tool from end to end;

5) Copy the length in 4) which could be 75.5878 mm or whatever;

6) Select both the linked file from 3) and the vector line from 4), then Object>Transform>Scale and insert the following in the Uniform scale box: hundred times the defining length, then /, then the copied length from 5), with a defining length of 100, it would be 10000/75.5878 (this will be 132.3017% but you have no use for the number);

Now you ought to have the right size/scale; you can check by clicking the scaled vector line and looking at the W og H box in the Transform palette;

7) Move the whole thing to fit your Artboard and delete/hide the vector line.

Then you can digitize in the right size.

If you want to scan your paper pattern in sections, you can combine the sections in Photoshop using File> Automate> Photomerge. 

Make sure everything is straight when taking the picture, a scan should not have that problem.

Measure something on the paper (you may make horizontal and vertical marks at a known distance), create an object the same size and scale the scan using that as a reference.

Just take a photo.

I'm not quite sure what exactly you're having issues with.