So back to my questions, How much do I give her, and what size needle?
You need a VET, not a horse breeder, not a friend, not a neighbor, not a barn owner, not a mortal enemy, A VET!!!
Don't know the number? GOOGLE IT! Don't have a chute, well woop de do, neither do I and the only time my Vet said I *had* to have one was when I wanted a wild cow treated. No transportation? No problem, Vets make house calls!
If your Vet doesn't do emergencies, get on the internet and find one that does. Not having the number of a Vet that is available 24hrs is highly irresponsible for a regular horse owner, downright INSANE for an owner of a bred mare.