I'm not sure about this, but wouldn't that change break the settings for everyone who has selected 10000,10000?
The user preference is stored as an offset in the database. The only way to fix this without a schema change and without breaking any existing user preferences seems to add new options at the bottom.
The following should work, but it's ugly (because it isn't sorted by size).

array (

array(320,240),
array(640,480),
array(800,480),
array(800,600),
array(1024,600),
array(1024,768),
array(1280,800),
array(10000,10000),
array(1280,1024),
array(1440x900),
array(1680x1050)
)

A nice fix would require a different way of storing this preference in the database.

giusca wrote:

I propose that the HD resolution (1920×1080) to be added as a default download resolution, as that's a common resolution for all kinds of displays.

See also T71973: Allow bigger thumbnail sizes in personal settings.