I absolutely love these arm warmers!  I use them for running, and use them when its too warm to wear a long sleeve shirt but too cold to not have long sleeves.  With these I can easily take the sleeves off when I get too warm, which I love.  (For me that's about 40-50*F.) I am 5'5"/155 lbs and usually order medium size shirts so I ordered a medium and a large to see which would fit best.  I couldn't even get the mediums past my elbows, and I do not have built arms.  I highly recommend that you order a size (or two!) larger than you're thinking.

My only complaint is that they only come in white or black.  I'd like a few more colours so I would know which I've already worn during the week.