Hello,

You are right, it's quite impossible to make a reliable estimation.
That being said, you have to enter the logic that your interviewer is not interested in the answer, sometimes he doesn't even know the answer. This is a perfect example of when you interviewer wants just to test your problem solving skills and see how you work under pressure.

In these cases, you have to:

1. Deline a strategy and expose it to your interviewer, making clear each step and being really careful to his potential inputs
2. Estimate each number doing an educate guess, everytime involving the interviewer in your reasoning
3. Do the calculations

Point 2 is crucial and it doesn't matter what is the number that you propose: it has only to be justified in a rational way.

Hope it helps,
Luca

I addition to Alexander's reply, I might add my 2 cents:

1. you can quickly estimate the row spacing by using the height of an average human (pictured) and then
2. eyeball the size of a leaf (in case you've never seen one in real life) and from there estimate the size of the grape bunch in relation to that
3. assume that grapes have a relatively similar density to water, and with the size of the bunch estimated you can then ballpark its weight

again, there are a few different ways to do this but it should be just done quickly so you can focus on the core problem.

Regards,
Radu