Most Helpful Guy
This is a tough question for all of us nice guys... I bet there is a low response rate to this question because most guys don't want to be truthful for fear of being flame-trolled to death for what they say. Because we are constantly admonished or accused of being shallow by the same machine that viciously and diligently keeps large people out of sight... because it's bad for business.
There is an imaginary line that every guy is aware of which, for him, represents what he can no longer find attractive no matter what other endearing qualities a girl/woman might possess to cancel out their repulsion threshold for larger size. Personally for me, the line is crossed when any of the errogenous body areas become large enough to become mishapen. Typically the tummy, thighs, butt, and even breasts will eventually reach a size where they no longer appear to be the proper shape. That happens before others areas such as chin, neck, or arms become mishapen.
As another male commentor mentioned above, this seems to happen at body weights above 180lbs with some +- for very tall or very short ladies.
For other men, regardless of their own body size, that line of unattractiveness could fall anywhere on the spectrum. Observation would indicate a typical line is drawn at body weights which exceed a man's own body weight. It is rare to see a man with a woman who is larger than he is. Not just by his preference but likely owing to the woman's preference as well: we like mates close to our own size.