My 10 y/o niece wanted legwarmers that go up past her knees. Kind of winged it, and they turned out to be a nearly perfect fit. Most importantly, she loves them.

CO 48 st with size 5. K2P2 rib 20 rows.
Switched to size 7, knit until measured 15” from CO.
K2P2 rib 20 rows with size 5, cast off.

Ran out of the TLC Essentials yarn with about 10 rows to go on the second legwarmer and hadn’t been able to find more, so I switched to the Red Heart to finish up. It didn’t feel as nice and was slightly shinier, but wasn’t too obvious because of the ribbing.