If I was fat, and now I'm not, which is better: 我长胖 or 我长胖了 or 我长了胖 or 我长了胖了?
Neither.

If you used to be x and you're not any more you'd just use a construct like:


  以前很x


like:


  (我)以前很胖




Of course, 很 doesn't necessarily mean really or very, but if you want to change it to 有点 that's cool too:


  (我)以前有点(儿)胖