[mdjlucan]

This is the first year, I've not a bad crop of grapes  but why are they so small?

[Aunt Sally]

Those vines have done well.

To produce bigger grapes you will need to thin them out, by at least a half, so that none of the young fruits are touching.

[Kleftiwallah]

Who said "These are small, but those are far away"!

Cheers,  Tony.

[mdjlucan]

What time is the best time to thin them out when they just start to grow?

[Yorkie]

The grape growing guidance on this site suggests in summer:
https://www.allotment-garden.org/fruit/grapes-growing/