I was given this plant in this pot 3 years ago. It is about 7 ft tall. The pot is 6-8". The plant is top heavy but healthy. How do I know if this plant should be repotted? What size pot would you recommend?
Should I Repot This Pencil Cactus?
Repotting every few years is always a good idea for the health of the plant, although some can do well without repotting for many years. Pencil cactus is one of these (obviously.) However, when your plant is so much bigger than its roots that it's falling over, that's a sign that it's repot time. (Other signs are plants that constantly wilt even though watered generously, and roots that are wound round and round into the pot shape when the plant is removed from the pot.)
When you repot this plant, you'll want to use a 10" pot, and a very free-draining potting mix. (My favorite is a 50-50 mix of cactus potting soil and perlite.) Also use a pot with drainage holes. Here are some more repotting tips: https://www.gardeningknowhow.com/houseplants/hpgen/learn-more-about-repotting-houseplants.htm
With such a large plant in such a small pot, it will be extremely important to loosen the roots as much as possible and stretch them out into the new soil. Also be especially careful about watering, making sure the new soil around the outside doesn't stay too wet.
Another thing you might want to do is double pot the plant, IOW set the newly potted plant into a larger, waterproof decorative container. This will also help to balance the size of the plant with its base.
By the way, your pencil cactus looks very nice and healthy.