There is no way to calculate the size of a Lightroom catalog file. There should be no problem keeping a catalog file and previews on your fastest disk, unless it is close to full right now.
Your photos ar not stored in the catalog itself, so just keep your 1.8 terabytes wherever they are right now.
Thanks for the answer
I was trying to find if 1.8 Terabyte of images would produce a catalogue, that would fit on a 256 gig SSD?
Maybe it would be better to create separate catalogues and store them in different locations?
Your advice is appreciated
Separate catalogs? Don't even consider it. Huge mistake.
The size of the catalog does not depends on the gigabytes or terbaytes of your photos. It depends on a lot of things, including the number of images and how much editing you do and how much metadata is assigned to these images and if you create books or web pages and the number of beers you drink when you are working on the catalog, and a couple of other things. But I sincerely doubt that it would come close to filling a 256GB SSD, its not something I would worry about.
I will purchase another SSD and catalogue away
Fuelled by a few whiskeys ..LOL
It is easy to monitor the size of your catalog. Locate the .lrcat file in use by Lightroom, if you are not sure where it is on your drive, you can find it here:
Catalog Settings / General / Location
As an example, my current catalog is referencing 54,000 images and uses 2.59 GB of storage.
Previews take up more space depending on the size of the previews associated with each file referenced. The same catalog of 54,000 images is using 26 GB total for both the JPEG previews and the Smart Previews.