Length – I generally recommend sizing sleep system length close to your total height or a couple inches longer. This leaves room for the top to cinch around your neck, allows for stomach sleepers to point their toes, prevents your toes from pushing into the loft at the footbox, and eliminates the likelihood of tension pulling on the shells.
For sizing Quilt Sleep Systems like the Coati, you will want to take a few measurements. The listed circumferences and widths for each system are taken at thirds along the length. Top / Torso / Knee / Foot.
Shoulders – the top of the system can be sized like the Hybrid Newt if desired. The most common size is anywhere from the measurement of you shoulder circumference, down to 8″ under your shoulder circumference. Anything above your shoulder circumference can be used if you desire extra space. It is often warranted to have some extra space with quilts to limit drafts.
Torso – Timmermade quilts come with a 8″ draft skirt that can be tucked under the sleeper in a similar function as the false bottom of the False Bottom Sleeping Bags. Therefore the width of the quilts can be sized similarly. If you plan on using fetal position read the measurement instructions for the fetal position false bottom bags. 
To measure for this, sit in a chair with thighs horizontal and knees together. Measure from the front of knees, down one thigh, around butt, up other thigh, and back to knees. Add an inch or two and this should give you a number that is large enough for your knees to pull up. The total width of Timmermade quilts is the width number plus 16″, which is the addition of the 8″ draft skirt on each side.
If you don’t plan on using your quilt in fetal position then it can be sized similar to the mummy shape systems. To measure the required torso area circumference for these systems you can simply loop a tape measure around your torso with room for your arms at your sides. Estimate the amount of room you need here. The total width of Timmermade quilts is the width number plus 14″, which is the addition of the 7″ draft skirt on each side.
Knee – This is less about a physical measurement around your knees than it is about estimating the amount of room you need to move here. The footbox is an area where you want to find a balance of efficiency vs comfort. Too much room means you’re building in a a lot of surface area to loose heat through, in a spot where you want efficient insulation around your extremities. Too little room and you won’t be able to move your legs and the whole system will want to spin with you when you roll. Run a tape measure around your knees to find out how much room is comfortable for you. Note that with the custom sizing option, you can stipulate a tapered footbox instead of the preset straight footbox.
Foot – To size the round foot end of the system, measure the length of your foot. Multiply that number by 3.14 and that will give you the circumference you need here. It is possible to get away with less since the circle can be elongated, but this is a safe starting point.