Here’s another way of looking at it. You’re a bicyclist, right? When I used to do fairly serious cycling (as a tourer, not a racer) I set my bicycle up so that the gears were as near to 10% apart as possible. On the size of rear block that I had, that was more or less one tooth difference between adjacent cogs.

A 24 is 4/20 bigger than a 20. That is 2/10, so a 24 is 20% bigger than a 20. That’s equivalent to about 2 gears on a touring cycle, or maybe 3 gears on a racing cycle. The difference will be noticeable, but not life changing.

I only put it in these terms because as a bicyclist, it might make some sense to you.

There is also the intimidation factor of a bigger wheel. However, the extra height is only half the difference, because the distance from axle to seat will be the same, and the distance from axle to ground is the *radius* of the wheel. So on a 24, you will be 2 inches higher than on a 20 - other things being equal.

And other things may not be equal. It is common to have 5 inch (125 mm) cranks on an off the shelf 20. It is common to have 6 inch (150 mm) cranks on an off the shelf 24. You set the seat height according to the distance from the bottom of the pedal travel, so if the crank is an inch longer, you have to set the seat an inch lower to be able to reach the pedal.

So, the 24 has a 4 inch bigger wheel, but you end up with a seat height only about 1 inch higher. That’s negligible.

Then there’s the control factor. A six inch crank is equal to half the radius of a 24 inch wheel. A 5 inch crank is equal to half the radius of a 20 inch wheel. Therefore, the leverage that you have is the same, so you have about the same amount of control. A 24 isn’t harder to pedal than a 20.

Some people find 150 mm cranks a bit long and unwieldy. You are a bicyclist used to (I guess) 170 mm cranks. You are also over 6 feet tall. Therefore you should have no difficulty with 150 mm cranks.

So what this boils down to is that you would notice very little difference between a standard 20 and a standard 24 when learning to ride.

Where it matters later is with things like sharp turns, and idling, where the bigger wheel is noticeably less manouvreable. It’s not a huge effect, but it’s there.

Remember that new cranks cost only a small amount (on a standard uni) and are easily swapped. The choice of tyre can make an inch or so difference to the effective wheel size. A pedals upgrade will give you more control, and so on. If you buy a 24, you can even buy a 20 inch wheelset to swap into the frame, rather than buying 2 unicycles.

In short, worry less.