How much memory a model needs
Memory is working space while a model runs. It is not the same as disk space for the download.
How the desktop app decides
The app compares on-disk file size with total memory on this computer. A file fits when it is no more than 60% of that memory. For example, a 4 GB file fits within an 8 GB memory check; a 5 GB file does not. If size or memory is unknown, the app cannot confirm a fit. Passing the check is not a promise of speed or that loading will succeed.
Rules of thumb
Smaller downloads usually need less memory and reply faster. Leave headroom for the app and your other work. If chat stops with a memory error, pick a smaller model from Discover with Fits my computer.
On the website
Set my machine on a model page for a site estimate with stated assumptions. A fit estimate there does not guarantee a successful load in the app.