When the subject (balls) is plural (meaning there are more than one) then you should use “are”.

You should not say “These ball are big” it should be “These balls are big”, because “these” is for plurals.

If there is only one ball, you should say “this ball”

If there is more than one ball, you should say “these balls”.

You should ask him Which ball is big?

These ball are big. These ball are small.

You should ask him Which balls are big?

This ball is big. These ball are small.

You should ask him Which ball is big?

These ball are big. This ball is small.

You should ask him Which balls are big?

the word ‘are’ denotes a plural subject (more than one): “The balls are big”

Your questions to him can be asked either way depending on whether you are asking about one ball or more than one ball.

use ‘are’ with ‘these’

whether it is affirmative, negative or

interrogative sentence.