I agree with Nicky on this one, sizes are definitely not consistent between brands.
I probably could have provided a link that could have articulated this better than I am about to, but I was feeling saucy and opinionated when I hit reply... so here goes.
As far as jeans are concerned, waist and length are still pretty much the 2 important factors. If you know your waist size and the length of jean that best fits you -- then your body type and preference of fashion will help determine the other intricacies.
For example, each size of jeans may come in: average, petite, short, medium, long, regular... etc.
Your typical "waist and length" measurement still applies, it's just that these will vary just a little bit in length.
The style of the jean can be: bootcut, flared, skinny, straight, tapered, wide... etc.
Your typical "waist and length" measurement still applies, these simply determine how fashionably tight or loose the jeans will wrap around your legs.
However the designer defines them, if there are different waist styles they are typically broken down into: above the waist, at the waist, below the waist, and extremely low. The "size and length" still applies, this just determines how much waist will be exposed while wearing those jeans.
It really isn't that complicated, it's just getting to know the new brands sizes. The best you can do is just try on a few of the jeans before your purchase. If they have an online site, check out their size chart before hand and possibly the availability if you're the least bit concerned they may not have your size in stock.
Happy shopping!