Short answer: 900 px if resolution (or quality) is set to medium.
Long answer . . .
Unfortunately when talking about pics on computers, size can mean two very different things so it can get confusing.
Meaning 1 is how much memory the file takes up.
This is measured in bytes or kilobytes, or KB.
Meaning 2 is how large it looks on your screen.
This could be measured in inches, mm or pixels, or px.
Here are two scenarios:
ONE
It is possible for a file to be forced to take up moderate memory, lets say 50 KB, but take up only one inch on the screen.
(This will look very high resolution or sharp.)
It is also possible to modify that same file so it takes up the same 50KB of memory, but appear a whopping 8" x 10" inches on your screen.
(This will look very low resolution or fuzzy, even though it appears larger.)
TWO
Next it is possible that the same pic can be forced to be low resolution (takes up only a little memory) and appear, say, 6" across on your screen.
This will look fuzzier than the next example.
Then the same file can be forced to be higher resolution (takes up more memory) but ALSO appear the exact same 6" across on your screen.
This will look sharper than the previous example.
PS only allows picture files that are smaller than 100 KB.
I find 900 px max will fit into this and fit on a standard monitor if the resolution (or quality) is set to medium on my software.