The color change in the water is due to the tannins being released from the wood. Mopani is a denser wood, so it generally sinks, but I'd still boil it. You can boil the wood at any time before or after placing it for a couple of instances. Boiling for 5-10 min on a high setting should take out most of the tannins, but these also don't harm the fish btw.

Each piece is unique but the mediums are generally appropriate for a 15 gallon tank or larger. I have three mediums in a 75g with a lot of room to spare, but an extra large is all that will fit in my 36g without overwhelming the tank, to give you an idea.

boil it for 5 - 10 minutes to clean the wood and prevent the water from getting too brown

no but it might get biofilm which doesn't harm ur fish just scrape it off

Yes but it may be too big, they are very nice. a 10 gallon with a 12 inch or so piece of wood is ging to look large in thre may want to get the small, its a good size as well.

If you just measured across it would 3 to 4 inches or wider in places, each one is different.

Thank you for your question. If you have had the driftwood in your aquarium for a year, it should no longer be effecting your water chemistry and more than likely something else is causing the pH.

These are heavy pieces that stay down even before soaking. No need to anchor them. They do have a lot of tannins that will leach into the water and turn it a tea color; so it's a good idea to boil the wood and/or soak it for (at least) several days in hot water until you notice that the water lightens up. I did this and mine still turned my water yellow. It doesn't bother me too much but I'm hoping with more water changes and new carbon in the filter that this will take care of it. The tannins won't hurt your fish, but I've read that it may slightly lower your PH and Water Hardness - still not a problem.