Yes, these are correct and reasonably clear. In real life, though, it is really important to be totally clear. That means avoiding situations where anyone needs to think hard about the precise meaning of words. Suppose you say "cut it back by one-third its size," and they misunderstand and cut it to one meter in height. At that point, it's too late to discuss the usage of "by" versus "to!" In real life, it would better to avoid these and say "Make it two meters high" or "make it one meter high." Better yet, you can say "I spray-painted an orange line on the tree. Cut it there." Or "here's a cell phone picture with my friend standing next to the tree. Make it as tall as my friend." Now that everyone can send pictures electronically, "a picture is worth a thousand words" is truer than ever. Even better yet is a sketch with a few short phrases written on it.

"cut back by 1/3 its size" = "cut back by 1/3" = "cut off 1/3 of it" = "cut off 1/3" = "remove 1/3 of it" = "take away 1/3 of it" Things become confusing when someone says something like this: "I am cutting that tree one third". It is impossible to know what that means because it can be interpreted either way.

Yep 👍