I’m knitting a sweater and it has to be 14 inches before beginning decreasing. I’ve got the back finished but I’m still not sure how to measure so i will get the front the correct size. Do I measure with the piece stretched slightly or should I make sure it’s relaxed. And do i count the stitches that are on the needle. When i measure, I always get a different measurement. What’ the easiest way to make sure the pieces are uniform in length?
Question about measuring length
I usually smooth out the piece and then lay the other piece on top of it and visually match up the edge stitches. Either that or count rows–how many V’s you have up to the decreases.
Lay it on a table and measure while relaxed. Unless you need to really block it out to get your width measurement. Count the stitches just under the neddle. The best way to make sure the pieces match in size is you make sure you have the same number of rows in each. Since you’ll be seaming up the sides, the rows need to match.
I know it is ideal if the rows match, but my gauge is not always perfect throughout and sometimes I have more stitches on one side. You can still seam it up with a little fudging and it looks fine. Sometimes my row gauge is perfect, and I can depend on the row count. 
I usually knit both front and back at the same time on a circ needle; then I do the same thing with the sleeves. Makes matching up the sides a breeze.
Ellie