Well I'm 5'8" and weighed about 180 (roughly 40 pounds overweight) when we first started TTC. We discovered that our problem was actually Male factor (my DH is very fit and lean so that didn't have anything to do with his issues). When I had our first IUI with a donor procedure done, I had only gotten down to 173ish. I got pg on the first try.
I do believe that if the body isn't in top notch shape, it can cause problems that makes getting pg harder but I don't think weight in itself is an issue. If you're overweight, have high blood pressure, diabetes, high cholesterol, etc. (things that can but don't always go along with being overweight) the body reacts to that and can make periods irregular and/or adjusts things that results in you not being as fertile as you would be if you were totally healthy (regardless of weight).