Need help with geometry buffer function

In the docs about the geospatial extension, it mentions a buffer function:

CREATE FUNCTION Buffer(a Geometry, distance FLOAT) RETURNS Geometry

Does anyone know what a good value would be to use for distance if I wanted to buffer a point, line, etc. X number of meters? Or does anyone have any ideas for what would be a good approach for this problem - where you want the user to be able to specify how close two things can be to one another to be a match?

Thanks a lot,

Casey