بالمكتب في مروحة صغيرة، بس بالبيت عندي مروحة كبيرة.

بالمكتب is made of:

• بـ = a very common preposition in Levantine, often meaning in, at, or with
• المكتب = the office

So بالمكتب literally means in the office or at the office.

A native English speaker may expect في المكتب, and that is possible too, but بالمكتب is very natural in everyday Levantine.

Here في is being used in its existential sense, meaning:

• there is
• there are

So:

• في مروحة صغيرة = there is a small fan

This is very common in Levantine.
So in this sentence, في is not really in. It is more like there is.

In Arabic, especially in the present tense, you often do not use a separate verb for is/are the way English does.

So instead of saying something like there is, Levantine can simply use:

• في for existence
• no present-tense to be in many noun/adjective sentences

That is why the sentence sounds natural without a word directly equivalent to English is.

بس here means but.

So:

• ... بس ... = ... but ...

In other contexts, بس can also mean only or just, but in this sentence it clearly means but because it contrasts the office and the home.

عندي literally means at me or with me:

• عند = at
• ـي = my / me

In Levantine, possession is often expressed this way:

• عندي سيارة = I have a car
• literally: at me, a car

So:

• بالبيت عندي مروحة كبيرة = at home, I have a big fan

This is a very normal way to say have in Arabic.

They are similar, but not exactly the same.

• بالمكتب في مروحة صغيرة = In the office, there is a small fan
• بالبيت عندي مروحة كبيرة = At home, I have a big fan

The first clause talks about simple existence in a place.
The second clause is more personal and speaker-centered: I have a big fan at home.

You could also say بالبيت في مروحة كبيرة, which would mean there is a big fan at home, but عندي adds the sense that it is part of what I have.

In Arabic, adjectives usually come after the noun they describe.

So:

• مروحة صغيرة = a small fan
• مروحة كبيرة = a big fan

This is the normal word order:

• noun + adjective

That is different from English, where adjectives usually come before the noun.

Because مروحة is a feminine noun.

In Arabic, adjectives must agree with the noun in:

• gender
• number
• definiteness

So since مروحة is feminine singular, the adjectives are also feminine singular:

• مروحة صغيرة
• مروحة كبيرة

If the noun were masculine, the adjective form would be different.

Because the sentence is talking about a small fan and a big fan, not the small fan and the big fan.

So:

• مروحة صغيرة = a small fan
• المروحة الصغيرة = the small fan

In Arabic, if the noun is indefinite, the adjective is also indefinite.
If the noun is definite, the adjective must also be definite.

So the matching is:

• مروحة صغيرة = indefinite + indefinite
• المروحة الصغيرة = definite + definite

Because the office and the house/home are specific places in this context, while the fan is not being identified as a specific known fan.

So:

• بالمكتب = in the office
• بالبيت = at home / in the house
• مروحة صغيرة = a small fan
• مروحة كبيرة = a big fan

This combination is very normal: a definite place, but an indefinite object inside it.

A common Levantine-style pronunciation would be something like:

bil-maktab fī mrawweḥa zghīre, bas bil-bēt ʿindī mrawweḥa kbīre.

A few helpful notes:

• بالمكتب → bil-maktab
• في → fī
• مروحة is often heard as mrawweḥa
• صغيرة is often pronounced zghīre
• كبيرة is often pronounced kbīre
• بالبيت → bil-bēt
• عندي → ʿindī

In Levantine, short vowels are often reduced, so the spoken form can sound more compact than the spelling suggests.

Yes, that is possible, and it would still be understandable.

But in natural Levantine speech, بالمكتب في مروحة صغيرة is very common and smooth. The speaker is using:

• بالمكتب = setting the location
• في مروحة صغيرة = saying that something exists there

So the original sentence sounds very natural conversationally.
Using في المكتب instead of بالمكتب is not wrong, but بـ with places is extremely common in Levantine.

Yes, Arabic allows some flexibility, especially in spoken dialects, but the emphasis may change.

For example:

• بالمكتب في مروحة صغيرة = In the office, there is a small fan
• في مروحة صغيرة بالمكتب = There is a small fan in the office

Both can work, but the original order sounds like the speaker is first setting up the location, then giving information about it.

So the sentence has a natural topic + comment feel:

• بالمكتب = topic/location
• في مروحة صغيرة = comment/new information