:( How do you determine the final size of your quilt? I want mine to be twin size, but everything I read confuses me for final measurments. stantard batting twin size is 72x90. Do I need to allow more than an extra 2 inches of batting to the final desired size?
Sorry I quess I didn't word this very well. I know what I want the final size to be, I just don't want to have to piece my batting. I will be attemping to machine quilt.

geez, I measure the "fall" and then across the bed, then add the "fall" again...

Then for the length, from the top edge of the bed to where I want it to end at the foot.

FWIW I also get batting on the roll, so I can afford to make it how wide I want, without consequence. If you're putting it on a frame, I would scoot the batting as close to the edge of your work as possible, because once you quilt there it's gonna stay put, kwim?

I allow 4 inches of extra batting to longarm quilt my quilts.

this is the way i do it too. when i did my DDs king, i had to really measure tight as my frame is not that long. what i didn't realize [still learning] is that the quilting takes up some of the length and width. so it came out 2" shorter. luckily, my DD didn't mind.

Your idea of twin size may not necessarily be the industry's standard idea. Measure and place a twin size flat sheet on the intended bed and decide if it's the size you envision. If not, add or subtract inches until it meets with your approval. Get a bigger batting if needed to allow a couple extra inches outside the top and backing.