How do I measure my shirt for the template in Design Space?

You'll want to measure your shirt exactly as the template measures it -- from sleeve to sleeve. Here's what a T-shirt template looks like in Cricut Design Space when you first select it:

To change the size of this measurement to match YOUR shirt, you choose Custom from the Size menu, which allows you access to the W (width) and H (height) fields, like this:

You'll note that right now, my W is 67.051 inches. If you look at your Canvas in Design Space, you'll see that this measurement is from sleeve to sleeve, but front and back, like this:

So to determine what size to adjust your template size, lay your T-shirt out on a flat surface like it is shown in the template, measure from sleeve tip to sleeve tip like this:

Then you just multiple that measurement by 2, because the Cricut template is for both the front and back of the shirt. Put the doubled measurement into the W box at the top of the screen. For example, in the photo above, the measurement from sleeve to sleeve is 30.5", so multiply that by 2 and you get 61". So we just change the W (width) to 61" on the template, like this:

To measure the height of your shirt, repeat the measuring process but measure from the highest point of your shirt (usually the top edge of the shoulder) to the hem. You do NOT need to double the measurement, of course, as the template also shows the measurement from the top to the bottom.