How pregnant is my cat?

I'm feeding a stray in my yard & have noticed past few days she's definitely pregnant. By the time I could tell, her sides looked pretty wide when looking at her from the front. I've tried since before this to get her inside- she comes in but freaks at any attempt to close the door. Now I'm afraid these kittens won't survive the cold while she's out hunting so I'd like to coax her in & force the situation by closing the door- she'll go nuts but maybe she'll calm down eventually. Problem- how can I tell how much time I have to accomplish this? She's a small cat but her sides are already pretty big- any info? ideas? advice on possible birth time? guidelines due date, relative to outward size?