Not only is Sally Field's part not big enough for a leading award, it's barely weighty enough for Supporting.
I think she will definitely be nominated for an Oscar, and possibly could win, but one of the major strikes against her is that she's short one or two substantial scenes to put her in a "lock." That has to do with the writing of the character.
It's not necessarily the size of the role, although that helps. Obviously people have won for five minutes of screen work, though. Sally doesn't have one of those scenes like Judi Dench did in Shakespeare in Love when it suddenly becomes "her movie" and the clocks stop on the walls while she's on screen. That's impact, and you can't judge it with a stopwatch.
Sally either needed more screen time or one of those takeover scenes. She has neither, even if she's damn good.
Tommy Lee Jones, on the other hand, has far more screen time AND several take-over-the-picture scenes.
"Jaws is the Citizen Kane of movies."
blocked: logan2, Diamonds3, Hamilton22