Variations. There are variations amount practices, but 40 mg of Methylprednisolone for a single injection is fairly standard.
Depends. An exact answer is not possible. There are many types of steroids, and the dosage will vary depending on the exact type of injection (there are different approaches to the nerves and the sac containing them). The part of the spine treated influences the choice of drug and amount too.
Depends. Every doctor uses a specific dose of steroid. Generally its hard to say what dose until you meet and speak with the doc. Also they may use different steroids and they are all very different in dosages.
Depends on the Drugs. Can't really answer this question briefly. Steroid drugs come in many strengths and some are several orders of magnitude stronger than others. But, for example, for lumbar tf esi (epidural steroid injection) the use of Dexamethasone 4, 8, or 12 mg each produce statistically significant improvement in "radicular" pain up to 12 weeks after injection. Good luck!