How can I see online collection sizes, and how does syncing work? Can't find this info anywhere.

Apparently you don't fully understand synchronization between LrC and Lightroom. IF you use Lightroom on your mobile phoone, and use the camera in Lightroom, it IS possible to have it configured so that when images are taken those images are immediately sent to Lightroom AND Lightroom Classic. And in the Lightroom Classic preferences it's possible to designate a folder where those images are to be stored. They will be stored in folders by date.

I'm sorry, but to say that you want an easier way to identify which images are full-sized and which are smart previews in Lightroom is kind of like pouting and saying, "I want my mommy!" You just have to do it. It seems to me from the way you have (not) managed your Lightroom online images you have created a mess and it's going to be up to you to clean it up. That's all there is to it. Nobody else has access to your account, so you are the one who has to go in and figure out what you want to do with it. The other alternative is to go into Lightroom Classic preferences And delete all of the synchronized data and start over. The choice is yours. There is a tool, can't remember what it's called, that you can download that would enable you to download everything that you have in the cloud. If you start another thread and asked that question someone else can answer it for you. I'm sure I've offended you. I won't bother you again. Good luck. I hope you get things figured out and straightened out

What is the source of your Lightroom Mobile originals? What are you using? Your mobile phone? Or, are you importing images directly into Lightroom for the cloud?

In LrC, have you designated a folder where your Lightroom images are added from the cloud? They should be added to Classic whenever they are added to Lightroom.

I illustrated how to identify originals/smart previews. For me, it isn't that difficult. I use a Nikon DSLR for most of my work and import those images into LrC. On rare occasions I will take a few images using my iPhone, and those are the ones that go directly to Lightroom. But I don't do that all that often.

In reality, from what I understand it isn't recommended to try to function with a mixed workflow using the two programs. It's especially frustrating trying to equalize the count between what you think should be the number of images in the cloud versus what Lightroom Classic indicates are in the cloud. Don't try to do it because it will only frustrate you. The synchronization process doesn't always work flawlessly. Do what you want, but it can be challenging to say the least.

In reality, if one image in an album is a smart preview it's likely that every image will be a smart preview. In thinking back on what I have done, it would be rare that there would be a mixture within an album. You're just going to have to bite the bullet and determine for yourself what you have and how you're going to clear things out. Nobody can do it for you.

What exact version of the program are you using? Frankly, I don't use Lightroom (the cloud version) that much for editing. I much prefer Lightroom Classic. However, when I tap the info button in Lightroom 4 this is what I see:

If you click on the info icon in the lower right-hand corner of the right-hand column there will be information indicating whether the image is a smart preview or the full-sized image.

The concept is fairly straightforward. Any images that you take with the Lightroom mobile camera on your phone, or import directly into Lightroom (the cloud version) send the full-sized images to the cloud, and they impact your cloud storage. Any images that you create in collections in Lightroom Classic and then share those collections do not impact your cloud storage because they are sent as smart previews. You can send as many collections from Lightroom Classic as you choose. If you have used up most of your allotted on storage and want more you have two choices. You can purchase additional storage from Adobe or you can delete full-sized images from the cloud. Deleting the full-sized images from the cloud will not remove them from copies that are in Lightroom Classic, so it would be possible to create collections using those images and share those collections. So what you are facing at this point is an issue of managing your cloud allotment.

As I explained  in my earlier post - Lightroom Classic can only sync smart previews via collections to the cloud and these will not be counted towards your cloud storage allocation. In other words, your cloud storage amount will not show any increase or decrease when they're add or removed.

>Also, how does syncing work? As per Adobe's online guide, syncing from cloud to computer downloads full versions, but syncing lightroom to cloud only uploads smart previews.

Lightroom Classic syncs a smart preview the cloud whereas Lightroom Desktop syncs the original file to the cloud.

Collections / Albums are virtual containers. As such, they have no physical content

>Does this mean when my full version is downloaded from the web, lightroom replaces the online version with a smart preview?

No, the original will remain on the cloud.

>As far as I can see, this seems to be the case, as my synced collections seem to be larger on my computer than the amount of space I'm using on the cloud.

 The smart previews sync to the cloud by Lightroom do not count towards you cloud storage allocation.