Here are two examples of size differences in the simple "round" caves from Plecocaves:
The short light brown-colored cave is a standard "round" cave. It's 6" long x 1 6/8" outer diameter (OD). It can be found here:
https://plecocaves.com/product/1-5-round-pleco-cave/.
The other cave (the "dark chocolate/almost black" according to the plecocaves website) is a larger round cave, 7" long x 1 7/8" outer diameter.
This 1/8" difference in OD makes a big difference in cave size.
I THINK the wider cave is this product:
https://plecocaves.com/product/1-5-round-cave-long/. This cave is listed as 7" long and 1.5" diameter. I think it's the right product because (A) the website doesn't list a 1 7/8" cave and (B) the next size up listed on the website is a full 2.25" (not 2") diameter... and I know I don't buy those.
For what it's worth, there's also a "longer" cave, which you can find here:
https://plecocaves.com/product/extra-lo ... ing-caves/. I've bought several of these.
They don't provide the slightly wider diameter, but I think they're about 9" long for the 1 1/2" diameter size.
Finally, you can try the Euro-D caves:
https://plecocaves.com/product/euro-d-pleco-cave/.
They list as 1 1/2" diameter, but the ones I received are slightly wider than the "round" caves or standard "D caves." When I look at the Plecocaves website, the only Euro-D caves I see have a crimped (slightly rounded) closed end (like the dark chocolate cave in my photo). I also possess a couple that have flat walled backs (like the light brown cave in my photo). I don't see them on the website, so maybe they aren't made anymore, but they add about 1/2-3/4" to the length of the caves.
Hope this helps.
Cheers,
Eric