Generally, no, tree topping isn’t a good technique, and you should avoid using it in most cases. Tree topping may be the only good option to save a tree following severe damage after a storm, but in most other cases, it’s not the right choice. Tree topping can leave trees vulnerable to disease and further damage, and most pros will suggest against it.

It depends. Smaller, younger trees take longer to mature, but they're more resilient, their roots don't get as damaged during transplanting, and they recover faster. Plus, they're much more affordable to plant, and it's possible to DIY the planting. 

On the other hand, mature trees produce fruit faster and provide more shade and privacy immediately. Ideally, a small or medium tree is the better choice if you can wait a few years for it to mature. But if you need immediate maturity, a larger tree is the only option.

In the case that a tree is growing directly on your property line, the cost of maintaining the tree is shared and is the property of both owners in common. In this case, all decisions regarding the maintenance of the tree must be made jointly, and both parties must agree on a course of action. You will need to agree with your neighbor about the cutting of any roots before you carry it out, and your neighbor must also consult with you before doing any trimming. Coming to an agreement with your neighbor about any shared trees ahead of necessary trimming and maintenance will make this process much simpler.

It is possible to cut down a tree on your property yourself, but it is generally not recommended unless specific conditions are met. You should only consider this project if the tree is small (e.g., under 4-5 inches in diameter and short enough that you don't need a ladder to reach the branches), and you are experienced in handling a chainsaw. The tree must also be located far from any power lines, fences, homes, or other structures. Even for small trees, the work can be dangerous, and it's always advisable to have a spotter to ensure the tree falls correctly and no branches fall on you.

For any tree that is large, requires a ladder to access, or is near electrical lines or structures, you should always hire a professional. Attempting to remove a large tree yourself is extremely dangerous, poses a risk of serious injury, and could result in significant damage to your property.

Before any tree removal, it's also important to check your local and state regulations to ensure the tree is not historic or otherwise protected by your city.