"Ruang saya tidak besar."

Translation: My room is not big.

[Aleksandro_RU]

What is the difference between ruang and kamar?

[mplisa]

They are have same meaning

[ObbieHadrian]

Harusnya "ruangan saya tidak besar"

[Kresimir809010]

Can it also be used in the sense of "my place isn't big", meaning "flat, apartment"?

[ObbieHadrian]

No one used "ruangan" for a private room, it is always for a public-ish space. When you want to express that you have small flat - apartment - or other private property, the word for it is "kamar".

Dining room - ruang makan
Living room - ruang keluarga
Bed room - kamar tidur

"my office (room) isn't big" - "ruangan (kantor) saya tidak besar" -- notice that the word implied in parentheses is different.
"my (apartment) room isn't big" - "kamar saya tidak besar"