How much would a metoprolol succinate extended release 25 mg lower heart rate? about how many bpm less?

This varies greatly from person to person. I have seen 12.5 mgs reduce a rate from 55 to 40 and I knew patients who took up to 100 mgs twice a day and heart rate stayed in the 60's. We start with low doses of beta blockers and work upwards according to heart rate & BP response.

This depends on information you did not give us. Are you of Chinese descent? How big are you? Why was it given? What other drugs are you taking? What is your salt intake? Do you have liver or kidney disease. What it your level of fitness? I would recommend you invest $20 and read uptodate.com for a week to see if you can tease out and anser as you do have this information. Discuss with YOUR Dr.