I am interested in getting a rabbit, specifically a Holland Lop rabbit, and am wondering how much cage room I should give the rabbit to live comfortably. This is just the cage space, I would also let the rabbit outside of its cage to play when I am there to supervise it.
This should be a good answer to your question: https://smallpetselect.com/how-much-space-does-a-rabbit-need/
Rabbits don't do well cooped up in a cage all day, but it isn't practical to supervise their antics 24/7. Rather than purchasing a "starter cage," invest in something big enough for your rabbit once they are full grown. Dog exercise pens are a terrific solution.
According to the House Rabbit Society, "one guideline to go by is at least 8 square feet of enclosure space combined with at least at least 24 square feet of exercise space, for 1-2 rabbits, in which the rabbit(s) can run and play at least 5 hours per day." In sum, the cage should be at least five times his size when stretched out; bigger is better!