I have a Honda prelude SI 1992 and I would like to know what is the size of the original tire rims?

I have a Honda prelude SI 1992 and I would like to know what is the size of the original tire rims? - 1

Go to any tire retailers website and enter year make and model of car while searching for tires. You will get the rim size and the tire size along with options. Hope that helps.
NOTE: Staying with original size gives the best combination of performance and mpg and handling. But if you are wanting to sacrifice pickup and handling you could go with a taller tire for more mpg but keep the width the same for traction purposes.
If you would rather have performance and want to sacrifice mpg and ride then go with a shorter tire and wider width for better traction and quick acceleration but lower top speed and mpg.

Look on the door sticker champ
If only the world could read instead of asking stupid questions

15x6 on a Si trim level Prelude.

Just like every other car on the road open the drivers door and read what it says on the sticker.