A telescope is defined by its collecting area. For an optical reflecting telescope, this is the diameter of its primary mirror. So how big can these mirrors get? Well, the York Observatory houses a telescope with a primary of 60 centimeters in diameter. I posted a picture of that mirror here when it was taken out for re-aluminizing. But mirrors can get a lot bigger than that. One of the largest mirrors in the world can be found at the Gemini Observatories (one in Hawaii atop Mauna Kea, and the other in Chile), which house telescopes with 8 meter primary mirrors. When the Gemini ‘scopes were being built, the National Optical Astronomy Observatory (NOAO) painted a circle on one of its outer walls the exact diameter of the primary mirrors of the new telescopes. Here’s me standing in front of that circle.
The largest optical reflecting telescope on the planet is the Gran Telescopio Canaris, in the Canary Islands. Here’s a list of the runner ups.