Since there's only about an inch difference in length, then go ahead with the longer measurement.
Lion and Red heart are two different sites. It's a Lion pattern
though. You'll need the amount given for the size 3, which is 800 yds of one color and 600 of the other. If you're going to make it all one color though, you probably need less altogether.
That seems like a lot of yarn, though, it would be over 1200 yds for a child's size 3 jacket? It does take more yarn for garter stitch, and if you use more colors, but for a single color I would expect it to only take about 600-800 yds at most.