One of the problems with the default gimp setup is it "remembers" last used settings and tools.
The default setting is actually one pixel. You must have entered 5000 sometime previously
What you want is 5 point 000
Just enter 5 Gimp will add the point 000
edit: guessing that is how you got the 1.
Swipe the value with the mouse to highlight. Then keyboard a 5 Use the little up-down arrows in the feather dialogue to adjust if required.
The default setting is actually one pixel. You must have entered 5000 sometime previously
What you want is 5 point 000
Just enter 5 Gimp will add the point 000
edit: guessing that is how you got the 1.
Swipe the value with the mouse to highlight. Then keyboard a 5 Use the little up-down arrows in the feather dialogue to adjust if required.