Angular
units are given as degrees, arc minutes, arc seconds.
An
arcsec is about the angular size of a quarter seen 5 km away.
The
resolution of your eye (which has a diameter of 2.5mm) at the wavelength of
visible light (which is 500 nm) is therefore:
2x10-4 x 5x102/2.5x10-3=40arcsec=2/3
arcmin
(the
Moon has an angular diameter of 30 arcmin).
To reach
the practical limit of what can be seen (~1 arcsec) through our
atmosphere, you need a telescope with 400 times that diameter, or 1
meter. Telescopes larger than that only gain you light gathering
power, unless you do something to reduce the blurring by the atmosphere. Radio
telescopes, which operate at wavelengths 10,000 times longer, will have far worse
resolution.