The height of the stand would depend on the height and width of the plant rather than ease of pruning. What is the plant and how big is it? Any chance of a photo?

Alternatively sit on the chair and see how far you can reach. But remember a plant needs pruning all round so doing it from one position is tricky.

Depends on the size of the pot and plant. If you are doing something small like a bonsai, you probably want a regular working table or desk height meant for sitting at. If you are doing bigger pots of something, you might want something shorter. Like if I have a pepper in a 3-5 gallon pot, I might use an old milk crate if I'm doing just one pot. I have used cinder blocks and wood planks to make a raised table for pepper pots. That's about 8-9" high. I use an upended 5 gallon bucket to sit on when working on my containers- those are the big 18-20 gallon totes.

My 8-inch to 2-foot tall plants are in 10-12-inch pots.

In years past, I would sit on the ground to prune the plants but now I'm disabled so I need to sit in a chair

I'm thinking about setting the potted plants on a appropriately-tall bar-stool (or similar stand) while I prune them

I was imagining much larger plants. If you have several plants how about shelves rather than individual stools. They can live on the shelves so no need to keep putting them on and off a stool.

I have about 30 potted plants surrounding my patio and about a dozen on a metal display stand with 3 tiers.

I can no longer kneel or sit on a kneeling-pad on the patio slab or reach the plants on the plant stand.

I'm thinking about buying a 28-to-32-inch tall "stool" and a turn-table.

I could then put one potted plant at a time on the turn-table on the "stool".

That would put the plant at eye-level for easy pruning and I could rotate the plant so I could to prune it "all around".

Sounds like a good solution. Let us know how it goes.