जो केक दुकानदार ने बनाया, उसका आकार बड़ा है।

This is a very common Hindi relative-correlative structure.

• जो केक दुकानदार ne बनाया = the cake that the shopkeeper made
• उसका आकार बड़ा है = its size/shape is big

Hindi often uses a two-part pattern like this instead of packing everything into one English-style relative clause. So literally it feels like:

The cake which the shopkeeper made — its size is big.

That is a normal Hindi way to connect the two ideas.

ने is the ergative marker. In standard Hindi, it is commonly used with the subject of a transitive verb in the perfective.

Here:

• verb: बनाया = made
• base verb: बनाना = to make
• this is a transitive action, and it is in the perfective

So Hindi says:

• दुकानदार ने बनाया = the shopkeeper made

Compare:

• दukानदार केक बनाता है = The shopkeeper makes cakes / is making a cake depending on context
  → no ने
• दुकानदार ने केक बनाया = The shopkeeper made the cake
  → ने appears

Because Hindi uses grammatical marking, not just word order.

• दुकानदार ने marks दुकानदार as the doer/agent
• केक is the thing being made

So even though English relies heavily on word order, Hindi gives you extra clues through markers like ने.

In this sentence:

• दुकानदार = the one who did the making
• केक = the thing that was made

Because in a perfective transitive clause with ने, the verb often agrees with the direct object, as long as that object is not marked with को.

Here the direct object is केक, which is being treated as masculine singular, so the verb is:

• बनाया = masculine singular

Compare:

• दुकानदार ने रोटी बनाई।
  The shopkeeper made bread/flatbread.
  (रोटी is feminine singular, so बनाई)

• दुकानदार ने केक बनाए।
  The shopkeeper made cakes.
  (masculine plural, so बनाए)

Because उसका agrees with the thing being possessed, not with the owner.

Here the possessed noun is:

• आकार = size/shape

आकार is masculine singular, so the possessive form is:

• उसका आकार = its size

Compare:

• उसकी सजावट = its decoration
  (सजावट is feminine singular)

• उसके रंग = its colors
  (रंग is plural)

So उसका is correct because आकार is masculine singular.

It refers back to जो केक... — in other words, the cake mentioned in the first part.

So:

• जो केक दुकानदार ने बनाया = the cake that the shopkeeper made
• उसका आकार = that cake’s size

This is the matching pair in the relative-correlative structure:

• जो ↔ उसका

Because बड़ा describes आकार, and आकार is masculine singular.

So:

• आकार बड़ा है = the size is big

If the noun were feminine, the adjective would change:

• सजावट बड़ी है = the decoration is big/grand

This is normal adjective agreement in Hindi.

Yes, that is a very common question.

जो is the basic relative form. It is often used directly before a noun:

• जो केक दुकानदार ने बनाया...

जिस is an oblique form and is usually used when a postposition is involved, or in structures like:

• जिस केक को दुकानदार ने बनाया... = the cake that the shopkeeper made
• जिस केक का रंग लाल है... = the cake whose color is red
• जिसमें... = in which

So in this sentence, जो keक... is perfectly normal. A more explicit version could be:

• जिस केक को दुकानदार ने बनाया, उसका आकार बड़ा है।

Both are acceptable, but the original is simpler and very natural.

Because Hindi does not have articles like the and a/an.

So केक by itself can mean:

• a cake
• the cake

The exact meaning comes from context.

In this sentence, the relative clause जो केक दुकानदार ने बनाया makes it clear that we are talking about a specific cake, so English naturally translates it with the.

Because direct objects in Hindi do not always take को.

With inanimate objects like केक, leaving out को is very common and often the default.

So this is normal:

• दुकानदार ने केक बनाया।

A version with a more explicit relative structure can use को:

• जिस केक को दुकानदार ने बनाया...

But in the original sentence, जो केक दुकानदार ने बनाया is already complete and natural.

No, not strictly.

The comma just helps show the pause between:

• the relative part: जो केक दुकानदार ने बनाया
• the main statement: उसका आकार बड़ा है

You may also see it written without a comma:

• जो केक दुकानदार ने बनाया उसका आकार बड़ा है।

Both are fine.

Yes.

Adding है after बनाया gives the relative clause a more present-perfect feel:

• बनाया = made
• बनाया है = has made / has made and it is relevant now

So both are possible:

• जो केक दुकानदार ने बनाया, उसका आकार बड़ा है।
• जो केक दुकानदार ने बनाया है, उसका आकार बड़ा है।

The version without है is still completely normal and often sounds a little lighter or more straightforward.