Hah, I thought I was being overly obvious. BiG measurements are the standard way of measuring jeans. Check that out. Then measure your thigh where they measure the thigh on jeans. Then, whenever you look at the measurements for a pair of jeans, it'll be pretty easy to tell how slim they'll fit your thighs when new. Then make a bit of an allowance for them stretching out and you're set. Really every newbie here thinks they have big thighs.