Anyone knows how to estimate the weight of a pitaya fruit (dragon fruit), where length and diamter is given?

Answer 1

First you have calculate its volume (look for formula in geometric handbook). Then you looking for its density in special literature (for instance, "hand guide to fruit's density"). And then calculating the unknown by multiplying its weight on its volume. Good luck!

Answer 2

I think you will be hard pushed to do it "the proper way"...!
The geometric shape is highly irregular, and I wonder where you can find a "hand book on fruit density".
You said "estimate", and therefore you can assume that the dragon fruit is mostly water. So just weigh the fruit.
But if you want to be more diligent than that:
Fill a container with water up to the rim.
Have it stand inside an empty container (weigh it first!)
Put the fruit into the water.
Weigh the water that spilled over into the empty container.
Assume water density to be "1" so you can calculate the volume of the fruit, and take it from there....