On the Downloads page, I note that it says:
For Ultimate editions we recommend a minimum of 6 GB of RAM and 20 GB of Storage Space.
20 GB may be too low a minimum size if the iso is not refreshed frequently, resulting in a large accumulation of updates. And don't forget you have set Timeshift to make a snapshot before an update takes place.
My experience: I downloaded KDE Ultimate to test in Virtualbox. This iso is dated 31 Aug 2020, which is 3 weeks ago. I set the VM storage size at around 25 GB Storage.
On first bootup, IIRC, the conky indicated a figure of around 17GB as used storage.
Upon initiating the first update after installation, I noted that the download size for the updates was HUGE (I think it was 6GB - I remember being very surprised) and wouldn't leave much breathing space.
I decided to remove Thunderbird, Wine, Proton packages (pamac indicated the wine and proton stuff were the largest packages installed) before commencing the update. I set timeshift to only keep one snapshot.
Even then, after this first update, I'm currently down to just 3+ GB free space. That is uncomfortably tight. If I had set the VM size at 20GB, I'm not sure it would have been enough space for that first round of updates.
Not sure what is going to happen when I do the second round of updates. When I have time that is. Or who knows, I may just wipe the VM.
I did a second VM install, this time on my laptop. I gave it 40GB storage, didn't remove any packages but limited timeshift to 1 snapshot. Much better breathing space.
I think it will be safer to recommend a minimum size of at least 30-35GB for KDE Ultimate.