If you have ring, take the vernier calliper gauge and just check the inside diameter of ring. Then using this table check your size.
If you do not have rings, you just take a thread and wrap around you finger. Then, check the length of thread/ When you have the length, you can calculate inside diameter using this formula: length of thread/3.14 = inside diameter. Then using this table check your size.
Yes, it is unfortunate but all silver will tarnish over time, especially this very well polished and shiny. Though it is easy to look after your silver jewellery at home to keep it looking clean and shiny longer. You can simply clean your silver jewellery using special cloths or putting it in special liquid substances (you can find these items nearly in all bigger shops)