Complex sentence: The phone connector was invented for use in telephone switchboards in the 19th century and is still widely used, both in its original inch (exactly 6.35 mm) size and in miniaturized versions: 3.5 mm (approx. inch) and 2.5 mm (approx. inch).
Split into simpler sentences: The phone connector was invented for use in telephone switchboards in the 19th century and is still widely used. The "mini" connector has a diameter of 3.5 mm (approx. inch) and the "sub-mini" connector has a diameter of 2.5 mm (approx. inch).