1) What is the average litter size?
I'm not sure what the average litter size is, but in all the years I've fostered cats and worked at the animal shelter, I've seen as few as one and as many as eight in a litter. I think the "norm" is probably three to five though. My three foster girls this year had four and five.
2) how big would the kittens be at birth?
I just found this quote on a website... "100 grams (3-5 ounces) at birth and gain approximately 7 to 15 grams per day (a quarter to over half an ounce)".
3) do i need to have a litter tray near to where she nests?
There should be one in the same room as the momma cat, but if you put it too close to the nest she will probably not have the babies there.
4) i noticed a slight swelling at her personal bits, is this normal?
Thats perfectly normal, and a sign she's getting close. My first two fosters this year had the swelling around 2 days before giving birth and the third had it exactly one week before giving birth.
5) she has started nesting under the bed, at what stage of the pregnancy is this?
I never actually saw any of my cats "nest"... they just didn't really do it. But I know it's a sign of being pretty close (probably within a week or two?). Have you felt babies move yet?
6) when she is giving birth will it be obvious (eg will it wake us up)?
Probably not. When my girls lost their mucus plugs, I just sat up with them and watched them in case any assistance was needed... they all gave birth in the middle of the night. Sasha (the first one that delivered), did let out a yell when the first kitten was coming out, but after that all was quiet and neither of the other cats made a sound. If I had been sleeping when they started, I probably wouldn't have woken up. You may hear the kittens screaming though.