Measure both palm circumference and length from base of hand to tip of fingers. The measure in inches is your size. If the two measures are very different, you'll need to decide which is more important.
It depends a little on how you plan on wearing it. If the idea is to use it as a middle layer under a coat/jacket, then 42 should work. If the idea is to use it as top layer on top of a chunky sweater, then I would go 44.
Unless you're planning on layering thick underneath (which I'm not sure works well for this style of jacket), I'd stick with 54 there. Maybe we still have it in the tobacco brown - the color is fantastic IRL.