riddle - Twins. Who are they?

You can find both in Istanbul, but none in water
They are the same size, but one of them is smaller

In some fields they can cause confusion
But if one grows, it will break the illusion

Different names, if called with the tone
But probably the same, if carved in stone

Answer

Is it

A capital i (I) and a lower case l (L)?

You can find both in Istanbul, but none in water

Both letters are in Istanbul, but neither in water

They are same size, but one of them is smaller

They are both full height, but the capital I is a bit smaller (Thanks @jwfrench !).

In some fields they can cause confusion

They look very similar in a lot of fonts

But if one grows, it will break illusion

If the lower case l is capitalized it no longer looks similar.

Different names, if called with the tone

Obviously called different and have different sounds

But probably same, if carved in the stone

A bit of a stretch here: because they look similar in text?