I had an 18" dishwasher for many years.

I would not do it again.

For the times when you really need a dishwasher, it was too small and required multiple loads.

And selection is non-existent compared to the full-sized models.

Miele has a 18" model with higher capacity than some older models. It will work for all youur everyday needs. Holidays often require two loads even for a full size.

My grandma's 18 in. Miele. No problems with washing and drying. Capacity is adequate for two people.

Here is a link that might be useful: slide show

Have you thought about a dish drawer? Same width but half the height?

If you go for 24" but want to do small loads, look for a regular washer that has a top-drawer only option.

I had to make the same decision in my apartment kitchen. there was no DW. In my case I had two options

1 keep double sinks wit 36" sink base and get a 18" DW nesxt to it then still have the same counter area too.

2 get smaller 1 1/2 basin or single basin sink in a 27" sink base, a 24" DW and 3" end panel. I got about 6' more counter to use but smaller sink.

I chose #2
the reason was I was worried about not having enough space to wash everything when there are dinner guests. I use the DW everyday and don't need a big sink, more usable counter if only inches in the same area was a plus. 18" machines that had style and options I wanted were $800-900 24' under $600. yeah I spent more changing the sink but I was getting new counters and faucet so it wasn't a lot more to do.

The first question is why do you want or need a smaller dishwasher.

What are you trying to accomplish by using one ?

Thanks for all the responses. The reason I'm thinking of a smaller dishwasher is because my island will be an L shape. I'd like to fit a double basin sink, dishwasher, microwave and trash pullout inside the L if that's even possible.

Currently, I have a triangular island with a double basin sink in the inner triangle part and a dishwasher to the right inside the triangle. My microwave is the hood over my range, and the trash bin is a stand alone.

I'm doing a new hood with no microwave and I don't have extra counter space for it. I really enjoy the double basin sink but I'd be willing to do a single if I could fit everything in. It's my only sink, not a prep sink. Also, I prefer a taller trash bin vs an under the sink short one. I store my cleaning supplies under my sink. I'd rather not do a stand alone again.

My new island will be approximately 4.5' x 5.5' on the inside, or just under that. One side faces my great room so I want to keep it clean, the other side faces the table and I'll do stools there.

Truth about an 18". It doesn't take as long to load or unload, perfect for us empty-nesters.
I put one in largely because it fit the current kitchen in an existing cabinet. We love it.
You will find the selection markedly reduced, and no savings on price i.e. the smaller size.
Your friends will find it fascinating as they probably have never seen one before.

I was going to do an 18 inch dishwasher, and I looked at both GE and Miele models. The Miele one seemed to hold a lot more than the GE one, and I felt I could easily get by with an 18 inch dishwasher on a day-to-day basis and for semi-frequent entertaining of 6-10 guests.

I also have a double sink, and I hand-wash most of my pots and pans. My goal for my kitchen was maximizing storage, and it seemed silly to store dirty dishes in a dishwasher the expense of clean ones in the dish drawer next to the dishwasher.

However, in the end, I got a 24 inch model because of some changes in the configuration of cabinets and a great deal on the DW.

One thing I've noticed is that I frequently have to run my DW without it being totally filled up because it starts to smell after a few days. If I had the 18 inch DW, I might not have this issue since I would, presumably, fill it up more quickly.