Which chain should I use for Rex Touring with 5 gears? The bike is about 15 years old. The size of tyre is probably 28.

I want to change the old chain.
I am not sure which chain I can put to the bike.

Does any chain which length is enough for 28 fit my bike?
To directly answer your question, no, any chain which is long enough is not compatible with your bike. No one here can tell you what chain will fit without a lot more detail in your question, but this is a good place to start. Daniel posted that link as a comment on my earlier answer.

Ask your LBS. They will know once they see the bike. Or measure the width of the outside chain plates with a vernier caliper on the old chain.