多大【duō dà】

多大 asks someone's age. Literally it is "how big", and the same frame – 多 plus an adjective – asks about any measurable quality.

No 是 appears. nǐ你shì是duō dà多大 is wrong.

Which form for whom

几岁 for a small child, because the answer is a single digit. 多大 for a teenager or an adult you can be casual with. 多大年纪 or 多大岁数 for someone older, where a blunt 你多大 would be rude.

Answering

The number carries 岁, and again no verb.

多 plus other adjectives

The same pattern measures anything: 多高, 多远, 多长, 多重.